will suffice. It explains the theory of the Photographic processes of
Daguerre and Talbot; especially noticing those points in which they may be
contrasted with Photography upon Collodion, but omitting all description
of manipulatory details, which if included would extend the Work beyond
its proposed limits.
The title of the second principal division of the Work, viz. "The practice
of Photography upon Collodion," explains itself. Attention however may be
invited to the fifth Chapter, in which a classification is given of the
principal imperfections in Photographs, with short directions for their
removal; and to Chapter VI., which describes the preservation of the
sensitiveness of Collodion plates and the mode of operating upon films of
Albumenized Collodion.
In Part III. will be found, in addition to a statement of the laws
of chemical combination, etc., a list of Photographic chemicals,
alphabetically arranged, including their preparation and properties as far
as required for their employment in the Art.
The reader will at once gather from this sketch of the contents of the
volume before him, that whilst the general theory of every Photographic
process is described, with the preparation and properties of the chemicals
employed, minute directions in the minor points of manipulation are
restricted to Photography upon Collodion, that branch of the Art being the
one to which the time and attention of the author have been especially
directed. Collodion is allowed by all to be the best vehicle for the
sensitive Silver Salts which is at present known, and successful results
can be obtained with a very small expenditure of time and trouble, if the
solutions employed in the process are prepared in a state of purity.
CHAPTER I.
HISTORICAL SKETCH OF PHOTOGRAPHY.
The Art of Photography, which has now attained such perfection, and has
become so popular amongst all classes, is one of comparatively recent
introduction.
The word Photography means literally "writing by means of Light;" and it
includes all processes by which any kind of picture can be obtained by the
chemical agency of Light, without reference to the nature of the sensitive
surface upon which it acts.
The philosophers of antiquity, although chemical changes due to the
influence of Light were continually passing before their eyes, do not
appear to have directed their attention to them. Some of the _Alchemists_
indeed noticed the fact that a substance which they termed "Horn Silver,"
which was probably a Chloride of Silver which had undergone fusion, became
_blackened_ by exposure to Light; but their ideas on such subjects being
of the most erroneous nature, nothing resulted from the discovery.
The first philosophical examination of the decomposing action of Light
upon compounds containing Silver was made by the illustrious Scheele, no
longer than three-quarters of a century ago, viz. in 1777. It was also
remarked by him that some of the coloured rays of Light were peculiarly
active in promoting the change.
_Earliest application of these facts to purposes of Art._--The first
attempts to render the blackening of Silver Salts by Light available for
artistic purposes were made by Wedgwood and Davy about A.D. 1802.
A sheet of white paper or of white leather was saturated with a solution
of Nitrate of Silver, and the _shadow_ of the figure intended to be copied
projected upon it. Under these circumstances the part on which the shadow
fell remained white, whilst the surrounding exposed parts gradually
darkened under the influence of the sun's rays.
Unfortunately these and similar experiments, which appeared at the
outset to promise well, were checked by the experimentalists being
unable to discover any means of fixing the pictures, so as to render
them indestructible by diffused Light. The unchanged Silver Salt being
permitted to remain in the white portions of the paper, naturally caused
the proofs to blacken in every part, unless carefully preserved in the
dark.
_Introduction of the Camera Obscura, and other Improvements in
Photography._--The "Camera Obscura," or darkened chamber, by means of
which a luminous image of an object may be formed, was invented by
Baptista Porta, of Padua; but the preparations employed by Wedgwood were
not sufficiently sensitive to be easily affected by the subdued light of
that instrument.
In the year 1814, however, twelve years subsequent to the publication of
Wedgwood's paper, M. Niépce, of Chalons, having directed his attention to
the subject, succeeded in perfecting a process in which the Camera could
be employed, although the sensibility was still so low that an exposure of
some hours was required to produce the effect.
In the process of M. Niépce, which was termed "Heliography," or
"sun-drawing," the use of the Silver Salts was discarded, and a resinous
substance, known as "Bitumen of Judæa," substituted. This resin was
smeared on the surface of a metal plate, and exposed to the luminous
image. The light in acting upon it so changed its properties, that it
became _insoluble_ in certain essential oils. Hence, on subsequent
treatment with the oleaginous solvent, the shadows dissolved away, and
the _lights_ were represented by the unaltered resin remaining on the
plate.
_The Discoveries of M. Daguerre._--MM. Niépce and Daguerre appear at one
time to have been associated as partners, for the purpose of mutually
prosecuting their researches; but it was not until after the death of the
former, viz. in 1839, that the process named the Daguerreotype was given
to the world. Daguerre was dissatisfied with the slowness of action of the
Bitumen sensitive surface, and directed his attention mainly to the use of
the Salts of Silver, which are thus again brought before our notice.
Even the earlier specimens of the Daguerreotype, although far inferior
to those subsequently produced, possessed a beauty which had not been
attained by any Photographs prior to that time.
The sensitive plates of Daguerre were prepared by exposing a silvered
tablet to the action of the vapour of _Iodine_, so as to form a layer of
Iodide of Silver upon the surface. By a short exposure in the Camera an
effect was produced, not visible to the eye, but appearing when the plate
was subjected to the vapour of Mercury. This feature, viz. the production
of a _latent_ image upon Iodide of Silver, with its subsequent development
by a chemical reagent, is one of the first importance. Its discovery at
once reduced the time of taking a picture from hours to minutes, and
promoted the utility of the Art.
Daguerre also succeeded in fixing his proofs, by removal of the unaltered
Iodide of Silver from the shadows. The processes employed however were
imperfect, and the matter was not set at rest until the publication of a
paper by Sir John Herschel, on the property possessed by "Hyposulphites"
of dissolving the Salts of Silver insoluble in water.
_On a means of Multiplying Photographic Impressions, and other Discoveries
of Mr. Fox Talbot._--The first communication made to the Royal Society
by Mr. Fox Talbot, in January, 1839, included only the preparation of a
sensitive paper for copying objects by application. It was directed that
the paper should be dipped first in solution of Chloride of Sodium, and
then in Nitrate of Silver. In this way a white substance termed Chloride
of Silver is formed, more sensitive to light than the Nitrate of Silver
originally employed by Wedgwood and Davy. The object is laid in contact
with the prepared paper, and, being exposed to light, a copy is obtained,
which is Negative,--_id est_, with the light and shade reversed. A second
sheet of paper is then prepared, and the first, or Negative impression,
laid upon it, so as to allow the sun's light to pass through the
transparent parts. Under these circumstances, when the Negative is raised,
a natural representation of the object is found below; the tints having
been again reversed by the second operation.
This production of a Negative Photograph, from which any number of
Positive copies may be obtained, is a cardinal point in Mr. Talbot's
invention, and one of great importance.
The patent issued for the process named Talbotype or Calotype dates from
February, 1841. A sheet of paper is first coated with Iodide of Silver by
soaking it alternately in Iodide of Potassium and Nitrate of Silver; it
is then washed with solution of Gallic Acid containing Nitrate of Silver
(sometimes termed Gallo-Nitrate of Silver), by which the sensibility to
light is greatly augmented. An exposure in the Camera of some seconds or
minutes, according to the brightness of the light, impresses an invisible
image, which is brought out by treating the plate with a fresh portion of
the mixture of Gallic Acid and Nitrate of Silver employed in exciting.
On the use of Glass Plates to retain Sensitive Films.--The principal
defects in the Calotype process are attributable to the coarse and
irregular structure of the fibre of paper, even when manufactured with the
greatest care, and expressly for Photographic purposes. In consequence of
this, the same amount of exquisite definition and sharpness of outline as
that resulting from the use of metal plates, cannot be obtained.
We are indebted to Sir John Herschel for the first employment of glass
plates to receive sensitive Photographic films.
The Iodide of Silver may be retained upon the glass by means of a layer of
Albumen or white of egg, as proposed by M. Niépce de Saint-Victor, nephew
to the original discoverer of the same name.
A more important improvement still is the employment of "Collodion" for a
similar purpose.
Collodion is an ethereal solution of a substance almost identical with
Gun-Cotton. On evaporation it leaves a transparent layer, resembling
gold-beater's skin, which adheres to the glass with some tenacity. M. Le
Grey of Paris originally suggested that this substance might possibly be
rendered available in Photography, but our own countryman, Mr. Archer, was
the first to carry out the idea practically. In a communication to 'The
Chemist' in the autumn of 1851, this gentleman gave a description of the
Collodion process much as it now stands; at the same time proposing the
substitution of _Pyro_-gallic acid for the Gallic acid previously employed
in developing the image.
At that period no idea could have been entertained of the stimulus which
this discovery would render to the progress of the Art; but experience has
now abundantly demonstrated, that, as far as all qualities most desirable
in a Photographic process are concerned, none at present known can excel,
or perhaps equal, the Collodion process.
CHAPTER II.
THE SALTS OF SILVER EMPLOYED IN PHOTOGRAPHY.
By the term Salt of Silver we understand that the compound in question
contains Silver, but not in its elementary form; the metal is in fact in
a state of chemical union with other elements which disguise its physical
properties, so that the Salt possesses none of the external characters of
the Silver from which it was produced.
Silver is not the only metal which forms Salts; there are Salts of Lead,
Copper, Iron, etc. Sugar of Lead is a familiar instance of a Salt of Lead.
It is a white crystalline body, easily soluble in water, the solution
possessing an intensely sweet taste; chemical tests prove that it contains
Lead, although no suspicion of such a fact could be entertained from a
consideration of its general properties.
Common Salt, or Chloride of Sodium, which is the type of the salts
generally, is constituted in a similar manner; that is to say, it contains
a metallic substance, the characters of which are masked, and lie hid in
the compound.
The contents of this Chapter may be arranged in three Sections: the first
describing the Chemistry of the Salts of Silver; the second, the action of
Light upon them; the third, the preparation of a sensitive surface, with
experiments illustrating the formation of the Photographic image.
SECTION I.
Chemistry of the Salts of Silver.
The principal Salts of Silver employed in the Photographic processes are
four in number, viz. Nitrate of Silver, Chloride of Silver, Iodide of
Silver, and Bromide of Silver. In addition to these, it will be necessary
to describe the Oxides of Silver.
THE PREPARATION AND PROPERTIES OF THE NITRATE OF SILVER.
Nitrate of Silver is prepared by dissolving metallic Silver in Nitric
Acid. Nitric Acid is a powerfully acid and corrosive substance, containing
two elementary bodies united in definite proportions. These are Nitrogen
and Oxygen; the latter being present in greatest quantity.
Nitric Acid is a powerful solvent for the metallic bodies generally. To
illustrate its action in that particular, as contrasted with other acids,
place pieces of silver-foil in two test-tubes, the one containing dilute
Sulphuric, the other dilute Nitric Acid; on the application of heat a
violent action soon commences in the latter, but the former is unaffected.
In order to understand this, it must be borne in mind that when a metallic
substance dissolves in an acid, the nature of the solution is different
from that of an aqueous solution of salt or sugar. If salt water be boiled
down until the whole of the water has evaporated, the salt is recovered
with properties the same as at first; but if a similar experiment be made
with a solution of Silver in Nitric Acid, the result is different: in that
case metallic Silver is not obtained on evaporation, but Silver combined
with Oxygen and Nitric Acid, both of which are strongly retained, being in
fact in a state of chemical combination with the metal.
If we closely examine the effects produced by treating Silver with Nitric
Acid, we find them to be of the following nature:--first, a certain
amount of Oxygen is imparted to the metal, so as to form an _Oxide_, which
Oxide dissolves in another portion of the Nitric Acid, producing _Nitrate_
of the Oxide, or, as it is shortly termed, Nitrate of Silver.[1]
[1] The preparation of Nitrate of Silver from the standard coin of the
realm is described in Part III., Art. "Silver."
It is the instability of Nitric Acid therefore--its proneness to part with
Oxygen--which renders it superior to the Sulphuric and to most acids in
dissolving Silver and various other substances, both organic and inorganic.
_Properties of Nitrate of Silver._--In preparing Nitrate of Silver, when
the metal has dissolved, the solution is boiled down and set aside to
crystallize. The salt however as so obtained is still acid to test-paper,
and requires either re-crystallization, or careful heating to about 300°
Fahrenheit. It is this retention of small quantities of Nitric Acid, and
sometimes probably of Nitrous Acid, which renders much of the commercial
Nitrate of Silver useless for Photography, until rendered neutral by
fusion and a second crystallization.
Pure Nitrate of Silver occurs in the form of white crystalline plates,
which are very heavy and dissolve readily in an equal weight of cold
water. The solubility is much lessened by the presence of free Nitric
Acid, and in the _concentrated_ Nitric Acid the crystals are almost
insoluble. Boiling Alcohol takes up about one-fourth part of its weight
of the crystallized Nitrate, but deposits nearly the whole on cooling.
Nitrate of Silver has an intensely bitter and nauseous taste; acting as a
caustic, and corroding the skin by a prolonged application. Its aqueous
solution does not redden blue litmus-paper.
Heated in a crucible the salt melts, and when poured into a mould and
solidified, forms the white _lunar caustic_ of commerce. At a still higher
temperature it is decomposed, and bubbles of Oxygen Gas are evolved: the
melted mass cooled and dissolved in water leaving behind a black powder,
and yielding a solution, which is faintly alkaline to test-paper, from
the presence of minute quantities of Nitrite or basic Nitrite of Silver.[2]
[2] Nitrite of Silver differs from the Nitrate in containing less Oxygen,
and is formed from it by the abstraction of two atoms of that element; it
is described in the vocabulary, Part III.
THE CHEMISTRY OF THE CHLORIDES OF SILVER.
_Preparation of Protochloride of Silver._--The ordinary white Chloride of
Silver may be prepared in two ways,--by the direct action of Chlorine upon
metallic Silver, and by double decomposition between two salts.
If a plate of polished silver be exposed to a current of Chlorine Gas,[3]
it becomes after a short time coated on the surface with a superficial
film of white powder. This powder is Chloride of Silver, containing the
two elements Chlorine and Silver united in single equivalents.
[3] For the properties of the element "Chlorine," see the third division
of the Work.
_Preparation of Chloride of Silver by double decomposition._--In order
to illustrate this, take a solution in water of Chloride of Sodium or
"common salt," and mix it with a solution containing Nitrate of Silver;
immediately a dense, curdy, white precipitate falls, which is the
substance in question.
In this reaction the elements change places; the Chlorine leaves the
Sodium with which it was previously combined, and crosses over to the
Silver; the Oxygen and Nitric Acid are released from the Silver, and unite
with the Sodium; thus
Chloride of Sodium _plus_ Nitrate of Silver
_equals_ Chloride of Silver _plus_ Nitrate of Soda.
This interchange of elements is termed by chemists double decomposition;
further illustrations of it, with the conditions necessary to the proper
establishment of the process, are given in the first Chapter of Part III.
The essential requirements in two salts intended for the preparation of
Chloride of Silver, are simply that the first should contain Chlorine,
the second Silver, and that both should be soluble in water; hence the
Chloride of Potassium or Ammonium may be substituted for the Chloride of
Sodium, and the Sulphate or Acetate for the Nitrate of Silver.
In preparing Chloride of Silver by double decomposition, the white clotty
masses which first form must be washed repeatedly with water, in order to
free them from soluble Nitrate of Soda, the other product of the change.
When this is done, the salt is in a pure state, and may be dried, etc., in
the usual way.
_Properties of Chloride of Silver._--Chloride of Silver differs in
appearance from the Nitrate of Silver. It is not usually crystalline,
but forms a soft white powder resembling common chalk or whiting. It is
tasteless and insoluble in water; unaffected by boiling with the strongest
Nitric Acid, but sparingly dissolved by concentrated Hydrochloric Acid.
Ammonia dissolves Chloride of Silver freely, as do solutions of
Hyposulphite of Soda and Cyanide of Potassium. Concentrated solutions
of alkaline Chlorides, Iodides, and Bromides are likewise solvents of
Chloride of Silver, but to a limited extent, as will be more fully shown
in Chapter IV., when treating of the modes of fixing the Photographic
proofs.
Dry Chloride of Silver carefully heated to redness fuses, and concretes on
cooling into a tough and semi-transparent substance, which has been termed
_horn silver_ or _luna cornea_.
Placed in contact with metallic Zinc or Iron acidified with dilute
Sulphuric Acid, Chloride of Silver is reduced to the metallic state, the
Chlorine passing to the other metal under the decomposing influence of the
galvanic current which is established.
_Preparation and Properties of the Subchloride of Silver._--If a plate
of polished Silver be dipped in solution of Perchloride of Iron, or of
Bichloride of Mercury, a _black stain_ is produced, the Iron or Mercury
Salt losing a portion of Chlorine, which passes to the Silver and converts
it superficially into Subchloride of Silver. This compound differs from
the white Chloride of Silver in containing less Chlorine; the composition
of the latter being represented by the formula AgCl, that of the former
may perhaps be written as Ag{2}Cl(?).
Subchloride of Silver is interesting to the Photographer as corresponding
in properties and composition with the ordinary Chloride of Silver
blackened by light. It is a pulverulent substance of a bluish-black colour
not easily affected by Nitric Acid but decomposed by fixing agents such as
Ammonia, Hyposulphite of Soda, or Cyanide of Potassium, into Chloride of
Silver which dissolves, and insoluble metallic Silver.
THE CHEMISTRY OF IODIDE OF SILVER.
The properties of _Iodine_ are described in the third division of the
Work: they are analogous to those of Chlorine and Bromine, the Silver
Salts formed by these elements bearing also a strong resemblance to each
other.
_Preparation and Properties of Iodide of Silver._--Iodide of Silver may be
formed in an analogous manner to the Chloride, viz. by the direct action
of the vapour of Iodine upon metallic Silver, or by double decomposition,
between solutions of Iodide of Potassium and Nitrate of Silver.
When prepared by the latter mode it forms an impalpable powder, the colour
of which varies slightly with the manner of precipitation. If the Iodide
of Potassium be in excess, the Iodide of Silver falls to the bottom of the
vessel nearly white; but with an excess of Nitrate of Silver it is of a
straw-yellow tint. This point may be noticed, because the yellow salt is
the one adapted for Photographic use, the other being insensible to the
influence of light.
Iodide of Silver is tasteless and inodorous; insoluble in water and in
dilute Nitric Acid. It is scarcely dissolved by Ammonia, which serves
to distinguish it from the Chloride of Silver, freely soluble in that
liquid. Hyposulphite of Soda and Cyanide of Potassium both dissolve Iodide
of Silver; it is also soluble in solutions of the alkaline Bromides and
Iodides, as will be further explained in Chapter IV.
Iodide of Silver is reduced by Metallic Zinc in the same manner as the
Chloride of Silver, forming soluble Iodide of Zinc and leaving a black
powder.
THE PREPARATION AND PROPERTIES OF BROMIDE OF SILVER.
This substance so closely resembles the corresponding salts containing
Chlorine and Iodine, that a short notice of it will suffice.
Bromide of Silver is prepared by exposing a silvered plate to the vapour
of Bromine, or by adding solution of Bromide of Potassium to Nitrate of
Silver. It is an insoluble substance, slightly yellow in colour, and
distinguished from Iodide of Silver by dissolving in strong Ammonia and in
Chloride of Ammonium. It is freely soluble in Hyposulphite of Soda and in
Cyanide of Potassium.
The properties of the element Bromine are described in Part III.
CHEMISTRY OF THE OXIDES OF SILVER.
_The Protoxide of Silver_ (Ag O).--If a little Potash or Ammonia be added
to solution of Nitrate of Silver, an olive-brown substance is formed,
which, on standing, collects at the bottom of the vessel. This is Oxide of
Silver, displaced from its previous state of combination with Nitric Acid
by the stronger oxide. Potash. Oxide of Silver is soluble to a very minute
extent in pure water, the solution possessing an alkaline reaction to
Litmus; it is easily dissolved by Nitric or Acetic Acid, forming a neutral
Nitrate or Acetate; also soluble in Ammonia (Ammonio-Nitrate of Silver),
and in Nitrate of Ammonia, Hyposulphite of Soda, and Cyanide of Potassium.
Long exposure to light converts it into a black substance, which is
probably a Suboxide.
_The Suboxide of Silver_ (Ag{2}O?)--This substance was obtained by Faraday
on exposing a solution of the Ammonio-Nitrate of Silver to the action of
the air. It bears a relation to the ordinary brown Protoxide of Silver
similar to that which the Subchloride bears to Protochloride of Silver.
Suboxide of Silver is a black or grey powder, which assumes the metallic
lustre on rubbing, and when treated with dilute Acids is resolved into
Protoxide of Silver which dissolves, and metallic Silver.
SECTION II.
_On the Photographic Properties of the Salts of Silver._
In addition to the Salts of Silver described in the first Section of this
Chapter there are many others well known to chemists, as the Acetate of
Silver, the Sulphate, the Citrate of Silver, etc. Some occur in crystals
which are soluble in water, whilst others are pulverulent and insoluble.
The Salts of Silver formed by colourless Acids are white when first
prepared, and remain so if kept in a dark place; but they possess the
remarkable peculiarity of being darkened in colour by exposure to Light.
Action of Light upon the Nitrate of Silver.--The Nitrate of Silver is one
of the most permanent of the Silver salts. It may be preserved unchanged
in the crystalline form, or in solution in distilled water, for an
indefinite length of time, even when constantly exposed to the diffused
light of day. This is partly explained by the nature of the acid with
which Oxide of Silver is associated in the Salt; Nitric Acid, possessing
strong oxidizing properties, being opposed to the darkening influence of
Light upon the Silver compounds.
Nitrate of Silver may, however, be rendered susceptible to the influence
of Light, by adding to its solution _organic matter_, vegetable or animal.
The phenomena produced in this case are well illustrated by dipping a
pledget of cotton-wool, or a sheet of white paper, in solution of Nitrate
of Silver, and exposing it to the direct rays of the sun; it slowly
darkens, until it becomes nearly black. The stains upon the skin produced
by handling Nitrate of Silver are caused in the same way, and are seen
most evidently when the part has been exposed to light.
The varieties of organic matter which especially facilitate the blackening
of Nitrate of Silver are such as tend _to absorb Oxygen_; hence pure
vegetable fibre, free from Chlorides, such, for instance, as the Swedish
filtering-paper, is not rendered very sensitive by being simply brushed
with solution of the Nitrate, but a little grape sugar added soon
determines the decomposition.
_Decomposition of Chloride, Bromide, and Iodide of Silver by Light._--Pure
moist Chloride of Silver[4] changes slowly from white to violet on
exposure to light. Bromide of Silver becomes of a grey colour, but is
less affected than the Chloride. Iodide of Silver (if free from excess
of Nitrate of Silver) does not alter in appearance by exposure even to
the sun's rays, but retains its yellow tint unchanged. Of these three
compounds therefore _Chloride_ of Silver is the most readily acted on
by light, and papers prepared with this salt will become far darker on
exposure than others coated with Bromide or Iodide of Silver.
[4] The Chloride here spoken of is the compound prepared by adding a
soluble Chloride to a solution of Nitrate of Silver: the product of the
direct action of Chlorine upon metallic Silver is sometimes insensitive to
light.
There are certain conditions which accelerate the action of light upon the
Chloride of Silver. These are, first, _an excess of Nitrate of Silver_,
and second, _the presence of organic matter_. Pure Chloride of Silver
would be useless as a Photographic agent, but a Chloride with excess of
Nitrate is very sensitive. Even Iodide of Silver, ordinarily unaffected,
is blackened by light when moistened with a solution of the Nitrate of
Silver.[5]
[5] The reader will understand that the Acetate, Sulphate, or any other
soluble Salt of Silver, might be substituted for the Nitrate in this
experiment.
Organic matter combined with Chloride and Nitrate of Silver gives a still
higher degree of sensibility, and in this way the Photographic papers are
prepared.
_The blackening of Chloride of Silver by Light explained._--This may
be studied by suspending pure Chloride of Silver in distilled water,
and exposing it to the sun's rays for several days. When the process of
darkening has proceeded to some extent, the supernatant liquid is found to
contain _free Chlorine_, or, in place of it. _Hydrochloric Acid_ (H Cl),
the result of a subsequent action of the Chlorine upon the water.
The luminous rays appear to loosen the affinity of the elements Chlorine
and Silver for each other; hence a portion of Chlorine is separated, and
the white Protochloride is converted into the violet _Sub_chloride of
Silver. If an atom of Nitrate of Silver be present, the liberated Chlorine
unites with it, displacing Nitric Acid, and forming again Chloride of
Silver, which is decomposed in its turn. The excess of Nitrate of Silver
thus exerts an accelerating influence upon the darkening of Chloride of
Silver, by rendering the chain of chemical affinities more complete, and
preventing an accumulation of Chlorine in the liquid, which would be a
check to the continuance of the action.
_Action of Light upon organic Salts of Silver._--On adding diluted
Albumen, or white of egg, to solution of Nitrate of Silver, a flocculent
deposit forms which is a compound of the animal matter with Protoxide of
Silver, and is known as "Albuminate of Silver." This substance is at first
quite white, but on exposure to light it turns to a brick-red colour. The
change which takes place is one of _deoxidation_, the Protoxide of Silver
losing a portion of its Oxygen, and a Suboxide of Silver, the product
of the reduction, remaining in union with the oxidized Albumen. The red
compound may therefore be loosely designated as an Albuminate of Suboxide
of Silver.
_Gelatine_ does not precipitate Nitrate of Silver in the same manner as
Albumen: but if a sheet of transparent Gelatine be allowed to imbibe a
solution of the Nitrate, it becomes of a clear ruby-red tint on exposure
to light, and a true chemical compound of Gelatine, or a product of its
oxidation, with a low Oxide of Silver, is produced.
Caseine, the animal principle of milk, is coagulated by Nitrate of Silver,
and the red substance formed on exposing the curds to light may be viewed
as analogous in composition to the corresponding compounds with Albumen
and Gelatine.
Many other organic salts of Silver are darkened by light. The white
Citrate of Protoxide of Silver changes to a red substance, reacting with
chemical tests in the same manner as Wöhler's Citrate of Suboxide of
Silver, which he obtained by reducing the ordinary Citrate in Hydrogen
Gas. Glycyrrhizin, the Sugar of Liquorice, also forms a white compound
with Oxide of Silver which becomes brown or red in the sun's rays.[6]
[6] For further particulars on the action of light upon the Salts of
Silver associated with organic matter, see the Author's paper on the
composition of the photographic image, in the eighth Chapter.
SIMPLE EXPERIMENTS ILLUSTRATING THE ACTION OF LIGHT UPON A SENSITIVE LAYER
OF CHLORIDE OF SILVER ON PAPER.
In the performance of the most simple experiments on the decomposition
of Silver Salts by Light, the student may employ ordinary test-tubes,
in which small quantities of the two liquids required for the double
decomposition may be mixed together.
When however concentrated solutions are used in this way, the insoluble
Silver Salt falls in dense and clotted masses, which, exposed to the
sun's rays, quickly blacken on the exterior, but the inside is protected,
and remains white. It is of importance therefore in Photography that the
sensitive material should exist in the form of _a surface_, in order that
the various particles of which it is composed may each one individually be
brought into relation with the disturbing force.
Full directions for the preparation of sensitive Photographic paper are
given in the second division of this work. The following is the theory of
the process:--A sheet of paper is treated with solution of Chloride of
Sodium or Ammonium, and subsequently with Nitrate of Silver; hence results
a formation of Chloride of Silver in a fine state of division, with an
excess of Nitrate of Silver, the Silver bath having been purposely made
stronger in proportion than the salting solution.
_Illustrative Experiment No. I._--Place a square of sensitive paper
(prepared according to the directions given in the Second Part of the
work) in the direct rays of the sun, and observe the gradual process of
darkening which takes place; the surface passes through a variety of
changes in colour until it becomes of a deep chocolate-brown. If the Light
is tolerably intense, the brown shades are probably reached in from three
to five minutes; but the sensibility of the paper, and also the nature of
the tints, will vary much with the character of the organic matter present.
_Experiment No. II._--Lay a device cut from black paper upon a sheet
of sensitive paper, and compress the two together by means of a sheet
of glass. After a proper length of exposure the figure will be exactly
copied, the tint however being reversed: the black paper protecting the
sensitive Chloride beneath, produces a _white_ figure upon a dark ground.
_Experiment No. III._--Repeat the last experiment, substituting a piece
of lace or gauze-wire for the paper device. This is intended to show the
minuteness with which objects can be copied, since the smallest filament
will be distinctly represented.
_Experiment No. IV._--Take an engraving in which the contrast of light
and shade is tolerably well marked, and having laid it closely in contact
with the sensitive paper, expose as before. This experiment shows that the
surface darkens in degrees proportionate to the intensity of the light, so
that the _half_ shadows of the engraving are accurately maintained, and a
pleasing gradation of tone produced.
In the darkening of Photographic papers, the action of the light is quite
superficial, and although the black colour may be intense, yet the amount
of reduced Silver which forms it is so small that it cannot conveniently
be estimated by chemical reagents. This is well shown by the results of
an analysis performed by the Author, in which the total weight of Silver
obtained from a blackened sheet measuring nearly 24 by 18 inches amounted
to less than _half a grain_. It becomes therefore of great importance in
preparing sensitive paper to attend to the condition of the surface layer
of particles, the action rarely extending to those beneath. The use of
Albumen, Gelatine, etc., which will be explained in the eighth Chapter,
has reference to this amongst other advantages, and secures a better and
more sharply defined print.
CHAPTER III.
ON THE DEVELOPMENT OF AN INVISIBLE IMAGE BY MEANS OF A REDUCING AGENT.
It has been shown in the previous Chapter that the majority of the Salts
of Silver, both organic and inorganic, are darkened in colour on exposure
to light, and, by the loss of Oxygen, Chlorine, etc., become reduced to
the condition of _Sub_salts.
Many of the same compounds are also susceptible of a change under the
influence of light, which is even more remarkable. This change takes
place after a comparatively short exposure, and as it does not affect the
appearance of the sensitive layer, for some time it escaped notice: but
it was afterwards discovered that an impression, before invisible, might
be brought out by treating the plate with certain chemical agents which
are without effect on the original unchanged salt, but quickly blacken it
after exposure.
It is a remarkable fact that the Silver compounds most readily affected by
light alone, are not the most sensitive to the reception of the invisible
image. Thus, of Photographic papers prepared with Chloride, Bromide, or
Iodide of Silver, the former assume the deepest shade of colour under the
influence of the sun's rays, but if all be exposed _momentarily_, and
then removed, the greatest amount of effect will be developed upon the
Iodide paper. Iodide of Silver therefore is the salt commonly used when
sensibility is an object, but it should be noted that images nearly or
quite latent can be impressed upon many other of the compounds of Silver,
including those belonging to the animal and vegetable kingdoms.
_Experiments illustrating the Formation of an Invisible Image._--Take a
sheet of sensitive paper, prepared with Iodide of Silver by the method
given in the fourth Chapter of Part II., and having divided it into two
parts, expose one of them to the luminous rays for a few seconds. No
visible decomposition takes place, but on removing the pieces to a room
dimly illuminated, and brushing with a solution of _Gallic Acid_, a
manifest difference will be observed; the one being unaffected, whilst the
other darkens gradually until it becomes black.
_Experiment II._--A prepared sheet is shielded in certain parts by an
opaque substance, and then after the requisite exposure, which is easily
ascertained by a few trials, treated with the Gallic Acid as before; in
this case the protected part remains white, whilst the other darkens to a
greater or less extent.
In the same way, copies of leaves, engravings, etc. may be made, very
correct in the shading and much resembling those produced by the prolonged
action of light alone upon the Chloride of Silver.
The object of employing a substance like Gallic Acid to _develope_ or
bring out to view an invisible image, in preference to forming the picture
by the direct action of light, unassisted by a developer, is the _economy
of time_ thereby effected. This is well shown in the results of some
experiments conducted by M. Claudet in the Daguerreotype process: he found
that with a sensitive layer of Bromo-Iodide of Silver, an intensity of
light three thousand times greater was required if the use of a developer
was omitted, and the exposure continued until the picture became visible
upon the plate.
To increase the sensitiveness of Photographic preparations is a point
of great consequence; and indeed, when the Camera is used, from the low
intensity of the luminous image formed in that instrument, no other plan
than the one above described would be practicable. Hence the advancement,
and indeed the very origin, of the Photographic Art, may be dated from the
first discovery of a process for bringing out to view an invisible image
by means of a reducing agent.
The present Chapter is divided into three Sections:--first, the chemical
properties of the substances usually employed as developers;--second,
their mode of action in reducing the Salts of Silver;--third, hypotheses
on the action of light in impressing a latent image.
SECTION I.
_Chemistry of the various Substances employed as Developers._
Development is essentially a process of _reduction_, or, in other words,
of _deoxidation_. If we take a certain metal, we can, by means of Nitric
Acid, impart Oxygen to it, so that it becomes first an Oxide, and
afterwards, by solution of the Oxide in the excess of acid, _a salt_. When
this salt is formed, by a series of chemical operations the reverse of
the former it may be deprived of all its Oxygen, and the metallic element
again isolated.
The degree of facility with which oxidation as well as reduction is
performed, depends upon the affinity for Oxygen which the particular
metal under treatment possesses. In this respect there is considerable
difference, as may be shown by a reference to the two well-known metals,
Iron and Gold. How speedily does the first become tarnished and covered
with rust, whilst the other remains bright even in the fire! It is indeed
possible, by a careful process, to form Oxide of Gold; but it retains its
Oxygen so loosely that the mere application of heat is sufficient to
drive it off, and leave the metal in a pure state.
Silver, Gold, and Platinum all belong to the class of _noble_ metals,
having the least affinity for Oxygen: hence their Oxides are unstable,
and any body tending strongly to absorb Oxygen will reduce them to the
metallic state.
Observe, therefore, that the substances employed by the Photographer
to assist the action of the light, and to develope the picture, act by
removing Oxygen. The sensitive Salt of Silver is thus _reduced_, more or
less completely, in the parts touched by light, and an opaque deposit
results which forms the image.[7]
[7] These remarks do not apply to the vapour of Mercury employed as a
developing agent in the Daguerreotype. The chemistry of that process will
be explained in a separate Chapter.
The most important of the developers are as follows:-- Gallic Acid,
Pyrogallic Acid, and the _Proto_salts of Iron.
CHEMISTRY OF GALLIC AND PYROGALLIC ACIDS.
a. _Of Gallic Acid._--Gallic Acid is obtained from _Gall Nuts_, which
are peculiar excrescences formed upon the branches and shoots of the
_Quercus infectoria_ by the puncture of a species of insect. The best
kind is imported from Turkey, and sold in commerce as Aleppo Galls. Gall
Nuts do not contain Gallic Acid ready formed, but an analogous chemical
principle termed _Tannic Acid_, well known for its astringent properties
and employment in the process of tanning raw hides.
Gallic Acid is produced by the _decomposition and oxidation_ of Tannic
Acid when powdered galls are exposed for a long time in a moist state to
the action of the air. By boiling the mass with water and filtering whilst
hot, the acid is extracted, and crystallizes on cooling, on account of its
sparing solubility in cold water.
Gallic Acid occurs in the form of long silky needles, soluble in 100 parts
of cold and 3 of boiling water; they are also readily soluble in Alcohol,
but sparingly in Ether. The aqueous solution becomes mouldy on keeping,
to obviate which, the addition of Acetic Acid or a drop or two of Oil of
Cloves is recommended.
Gallic Acid is a feeble acid, scarcely reddening litmus; it forms salts
with the alkaline and earthy bases, such as Potash, Lime, etc., but not
with the oxides of the noble metals. When added to Oxide of Silver the
metallic element is separated and the Oxygen absorbed.
b. _Pyrogallic Acid._--The term _pyro_ prefixed to Gallic Acid implies
that the new substance is obtained by the _action of heat_ upon that body.
At a temperature of about 410° Fahr., Gallic Acid is decomposed, and a
white sublimate forms, which condenses in lamellar crystals; this is
Pyrogallic Acid.
Pyrogallic Acid is very soluble in cold water, and in Alcohol and Ether;
the solution decomposes and becomes brown by exposure to the air. It gives
an indigo blue colour with Protosulphate of Iron, which changes to dark
green if any Persulphate be present.
Although termed an acid, this substance is strictly _neutral_; it does not
redden litmus-paper, and forms no salts. The addition of Potash or Soda
decomposes Pyrogallic Acid, at the same time increasing the attraction
for Oxygen; hence this mixture may conveniently be employed for absorbing
the Oxygen contained in atmospheric air. The compounds of Silver and Gold
are reduced by Pyrogallic Acid even more rapidly than by Gallic Acid, the
reducing agent absorbing the Oxygen, and becoming converted into Carbonic
Acid and a brown matter insoluble in water.
Commercial Pyrogallic Acid is often contaminated with empyreumatic oil,
and also with a black insoluble substance known as _Metagallic Acid_,
which is formed when the heat is raised above the proper temperature in
the process of manufacture.
CHEMISTRY OF THE PROTOSALTS OF IRON.
The combinations of Iron with Oxygen are somewhat numerous. There are two
distinct Oxides which form Salts, viz. the Protoxide of Iron, containing
an atom of Oxygen to one of metal; and the Peroxide, with an atom and a
half of Oxygen to one of metal. As _half atoms_ however are not allowed in
chemical language, it is usual to say that the Peroxide of Iron contains
three equivalents of Oxygen to two of metallic Iron.
Expressed in symbols, the composition is as follows:--
Protoxide of Iron, Fe O.
Peroxide of Iron, Fe{2}O{3}.
The Proto- and Persalts of Iron do not resemble each other in their
physical and chemical properties. The former are usually of an apple-green
colour, and the aqueous solutions almost colourless, if not highly
concentrated. The latter, on the other hand, are dark, and give a yellow
or even blood-red solution.
The Protosalts of Iron are alone useful in Photography; but the following
experiment will serve to illustrate the properties of both classes of
salts:--Take a crystal of Protosulphate of Iron, and, having reduced
it to powder, pour a little Nitric Acid upon it in a test-tube. On the
application of heat, abundance of fumes will be given off, and a red
solution obtained. The Nitric Acid in this reaction imparts Oxygen, and
converts the _Proto_sulphate entirely into a _Per_sulphate of Iron. It is
this feature, viz. the tendency to absorb Oxygen, and to pass into the
state of Persalts, which makes the Protosalts of Iron useful as developers.
There are two Protosalts of Iron commonly employed by Photographers: the
Protosulphate and the Protonitrate of Iron.
a. _Protosulphate of Iron._--This salt, often termed _Copperas_ or _Green
Vitriol_, is an abundant substance, and used for a variety of purposes in
the arts. Commercial Sulphate of Iron however, being prepared on a large
scale, requires re-crystallization to render it sufficiently pure for
Photographic purposes.
Pure Sulphate of Iron occurs in the form of large transparent, prismatic
crystals, of a delicate green colour: by exposure to the air they
gradually absorb Oxygen and become rusty on the surface. Solution of
Sulphate of Iron, colourless at first, afterwards changes to a red tint,
and deposits a brown powder; this powder is a _basic_ Persulphate of Iron,
that is, a Persulphate containing an excess of the oxide or _base_. By the
addition of Sulphuric or Acetic Acid to the solution, the formation of a
deposit is prevented, the brown powder being soluble in acid liquids.
The Crystals of Sulphate of Iron include a large quantity of water of
crystallization, a part of which they lose by exposure to dry air. By a
higher temperature, the salt may be rendered perfectly _anhydrous_, in
which state it forms a white powder.
b. _Protonitrate of Iron._--This salt is prepared by double decomposition
between Nitrate of Baryta or of Lead and Protosulphate of Iron. It is an
unstable substance and crystallizes with great difficulty; its aqueous
solution is pale green at first, but very prone to decomposition, even
more so than the corresponding Sulphate of Iron.
SECTION II.
_The Reduction of Salts of Silver by Developing Agents._
The general theory of the reduction of metallic oxides having been
explained, it may be desirable to enter more minutely into the exact
nature of the process as applied to the compounds of Silver.
First, the Reduction of the Oxide of Silver will be taken, as the most
simple illustration; then that of Salts of Silver formed by Oxygen-acids;
and lastly, of the Chloride, Iodide, and Bromide of Silver containing no
Oxygen.
_Reduction of Oxide of Silver._--To illustrate this conveniently, the
Oxide of Silver should be in a state of solution; water dissolves Oxide
of Silver very sparingly, but it is freely soluble in Ammonia, forming
the liquid known as Ammonio-Nitrate of Silver. If, therefore, a little of
the Ammonio-Nitrate of Silver be placed in a test-tube, and solution of
Sulphate of Iron be added to it, immediately it becomes discoloured, and a
deposit settles to the bottom.
This deposit is metallic Silver, produced by the reducing agent
appropriating to itself the Oxygen previously combined with the metal. As
metallic Silver does not dissolve in Ammonia, the liquid becomes turbid,
and the metal subsides in the form of a bulky precipitate.
_Reduction of the Oxyacid Salts of Silver._--The term _Oxyacid_ includes
those salts which contain the Oxide of Silver intimately combined with
Oxygen-acids; as _e. g._ the Nitrate of Silver, the Sulphate, the Acetate
of Silver, etc.
These salts, soluble in water, are reduced by developing agents in the
same manner as Oxide of Silver, but more slowly. The presence of an
acid united with the base is a hindrance to the process and tends to
keep the oxide in solution, especially when that acid is powerful in
its affinities. To illustrate the effect of the acid constituent of the
salt in retarding reduction, take two test-tubes, the one containing
Ammonio-Nitrate, and the other ordinary Nitrate of Silver--a single drop
of solution of Sulphate of Iron added to each will indicate an evident
difference in the rapidity of deposition.
The precipitate of metallic Silver obtained by the action of reducing
agents upon the Nitrate, varies much in colour and in general appearance.
If Gallic or Pyrogallic Acid be employed, it is a black powder;[8] whilst
the salts of Iron, and especially the same with free Nitric Acid added,
produce a sparkling precipitate, resembling what is termed _frosted
silver_. Grape Sugar and many of the essential oils, such as the Oil of
Cloves, etc., separate the metal from Ammonio-Nitrate of Silver in the
form of a brilliant mirror film, and are often employed in silvering glass.
[8] Silver precipitated by Gallic or Pyrogallic Acid does not appear to be
free from organic matter, and probably contains also a small proportion of
Oxygen.
In remarking upon these peculiarities in the molecular condition of
precipitated Silver, it should be observed that the appearance of a metal
whilst in mass is no indication of its colour when in the state of fine
powder. Platinum and Iron, both bright metals, and susceptible of a high
polish, are dull and intensely black when in a fine state of division;
Gold is of a purple or yellowish brown; Mercury a dirty grey.
_Reduction of the Hydracid Salts of Silver._--By the term _Hydracid_ is
meant Salts of Silver which contain no Oxygen or Oxygen-acids, but simply
elements like Chlorine or Iodine combined with Silver. These elements are
characterized by forming acids with Hydrogen, which acids are hence called
_Hydr_acids. Hydrochloric Acid (HCl) is an example; so also is Hydriodic
Acid (HI).
The reduction of the Hydracid Salts requires to be discussed separately,
because it is evidently different from that already described; the
reducing agent tending only to absorb _Oxygen_, which is not present in
these salts. The explanation is as follows: When a Chloride of a noble
metal is reduced by a developer, _an atom of water_, composed of Oxygen
and Hydrogen, takes a part in the reaction. The Oxygen of the water passes
to the developer, the Hydrogen to the Chlorine.
To illustrate this, take a solution of Chloride of Gold, and add to it a
little Sulphate of Iron. A yellow deposit of metallic Gold soon forms,
and the supernatant liquid is found, by testing, to be acid from free
Hydrochloric Acid. The following simple diagram, in which however the
_number_ of the atoms concerned is omitted, may assist the comprehension
of the change.
[Illustration:
Compound Atom of Compound Atom Atom of
Chloride of Gold. of Water. Sulphate of Iron.
]
The symbol Au represents Gold, Cl Chlorine, H Hydrogen, and O Oxygen.
Observe that the molecules H and O separate from each other and pass in
opposite directions: the latter unites with the Sulphate of Iron; the
former meets Cl, and produces Hydrochloric Acid (HCl), whilst the atom of
Gold is left alone.
Hence there is no theoretical difficulty in supposing a reduction of
Iodide of Silver by a developer, if we associate with the Iodide an atom
of water to furnish the Oxygen. Unless the sensitive plate however has
been exposed to the light, the reduction does not readily take place;
nor can it be produced under any circumstances, with or without light,
when the whole of the free Nitrate of Silver has been washed away from
the plate. Pure Iodide of Silver is therefore unaffected by a developer,
and the compound which blackens on the application of Sulphate of Iron or
Pyrogallic acid is an Iodide with excess of Nitrate of Silver.
[Illustration:
Compound Atom of Compound Atom of Atom of
Iodide of Silver. Nitrate of Silver. Sulphate of Iron.
]
The mode in which a Salt of Silver, such as the Nitrate, soluble in
water, may act in facilitating the reduction of Iodide of Silver, is shown
in the preceding diagram, which corresponds closely with the last.
Notice that the compound atom of Nitrate of Silver contains a molecule of
Oxygen for the developer, one of Silver (Ag) for the separated Iodine, and
an atom of Nitric Acid (NO{5}), which is liberated, and takes no further
part in the change.
The chain of chemical affinities is more complete in this diagram than
in the last, where an atom of water only was present, the affinity of
Iodine for Silver being greater than that of Iodine for Hydrogen. Hence
it is possible that an excess of Nitrate of Silver may, by furnishing an
elementary basis for which Iodine has an attraction, assist in drawing off
that element, so to speak, from the original particle of Iodide of Silver
touched by light.[9]
[9] The reader must not suppose from the remarks which have been made in
this Section that images obtained by development consist invariably of
pure metallic Silver. It can be shown that such is not the case,--that the
process of reduction is in many cases suspended when a part only of the
Oxygen has been removed; and hence results a _subsalt_ similar to that
produced by the direct action of light upon organic compounds of Silver,
and differing in properties from metallic Silver. For further particulars
see the Author's Photographic researches in the eighth Chapter.
SECTION III.
_The formation and development of the Latent Image._
It was shown in the second Chapter that the continued action of white
light upon certain of the Salts of Silver resulted in the separation
of elements like Chlorine and Oxygen and the partial reduction of the
compound. We have also seen that bodies possessing affinity for Oxygen,
such as Sulphate of Iron and Pyrogallic Acid, tend to produce a similar
effect; acting in some cases with great energy and precipitating metallic
Silver in a pure state.
In forming an extemporaneous theory on the production of the latent
image in the Camera, it would therefore be natural to suppose that the
process consisted in setting up a reducing action upon the sensitive
surface by means of light, afterwards to be continued by the application
of the developing solution. This idea is to a certain extent correct, but
it requires some explanation. The effects produced by the light and the
developer are not so precisely similar that the one agency can always
be substituted for the other: an insufficient exposure in the Camera
cannot be remedied by prolonging the development of the image. In the
Photographic processes on paper it is indeed found that a certain latitude
may be allowed; but, as a rule, it should be stated that a definite time
is occupied in the formation of the invisible image, which may not be
shortened or extended beyond its proper limits with impunity. There is a
maximum point beyond which no advance is made; hence if the plate be not
then removed from the Camera, those portions of the image formed by the
brightest lights are speedily overtaken by the "half tones," so that, on
developing, an image appears without that contrast between lights and
shadows which is essential to the artistic effect. On the other hand, in
a case of insufficient exposure, the feeble rays of light not having been
allowed time to impress the plate, the half shadows cannot be brought out
on subsequent treatment with the developing agent.
A careful study of the phenomena involved in this part of the process
cannot fail to show that the ray of Light determines a _molecular_ change
of some kind in the particles of Iodide of Silver forming the sensitive
surface. This change is not of a nature to alter the composition or the
chemical properties of the salt. The Iodine does not leave the surface,
or there would be a difference in the appearance of the film, or in its
solubility in Hyposulphite of Soda.
The following diagrams may perhaps be useful in mechanically illustrating
what is meant by a molecular change.
Fig. 1 represents a compound molecule of Iodide of Silver, the component
atoms of which are closely associated.
Fig. 2. The same after the action of a disturbing force. The simple
molecules have not altogether separated, but they are prepared to do so,
touching only at a single point.
[Illustration: Fig. 1.]
[Illustration: Fig. 2.]
Now the effect produced on this combination by a developer is understood,
if we suppose that in the first case the affinity of the Iodine for Silver
is too great to allow of its separation; but in the second, this affinity
having been loosened, the structure gives way, and metallic Silver is the
result.
This hypothesis has the merit of simplicity, and is not opposed to known
facts; it may therefore for the present be received. The point however on
which a doubt must rest is--whether the molecular disturbance produced
by light upon Iodide of Silver leads to a reduction of that Salt by the
developer. No image can be produced on the application of Pyrogallic Acid
_unless the particles of Iodide are in contact with Nitrate of Silver;_
and hence it may be the Nitrate and not the Iodide which is reduced--that
is, the impressed molecule of Iodide may determine the decomposition of a
contiguous particle of Nitrate, itself remaining unchanged. This view is
supported to some extent by Moser's experiments, shortly to be quoted; and
also by the fact that the delicate image first formed can be _intensified_
by treating it with a mixture of the developing solution and Nitrate of
Silver, even after the Iodide has been removed by a fixing agent. The
following experiment will serve to illustrate this.--
Take a sensitive Collodion plate, and having impressed an invisible
image upon it by a proper exposure in the Camera, remove it to the dark
room, and pour over it the solution of Pyrogallic Acid. When the picture
has fully appeared, stop the action by washing the plate with water,
and remove the unaltered Iodide of Silver by Cyanide of Potassium. An
examination of the image at this stage will show that it is perfect in
the details, but pale and translucent. The plate is then to be taken back
again to the dark room and treated with fresh Pyrogallic Acid, _to which
Nitrate of Silver has_ been added; immediately the picture becomes much
blacker, and continues to darken, even to complete opacity, if the supply
of Nitrate be kept up.
Now in this experiment it is evident that the additional deposit upon the
image is produced from the Nitrate of Silver, the whole of the Iodide
having been previously removed. Observe also, _that it forms only upon
the image, and not upon the transparent parts of the plate_. Even if the
Iodide, untouched by light, be allowed to remain, the same rule holds
good;--the Pyrogallic Acid and Nitrate of Silver react upon each other and
produce a metallic deposit; this deposit however has no affinity for the
unaltered Iodide upon the part of the plate corresponding to the shadows
of the picture, but attaches itself in preference to the Iodide already
blackened by light.
This second stage of the development, by which a feeble image may be
strengthened and rendered more opaque, is sometimes termed "development
by precipitation," and should be correctly understood by the practical
operator.
_Researches of M. Moser._--The papers of M. Ludwig Moser 'On the
Formation and Development of Invisible Images,' published in 1842,
explain so clearly many remarkable phenomena of occasional occurrence in
the Collodion and paper processes, that no apology need be offered for
referring to them somewhat at length.
His first proposition may be stated thus:--"If a polished surface has
been touched in particular parts by anybody, it acquires the property of
precipitating certain vapours on these spots differently to what it does
on the other untouched parts." To illustrate this, take a thin plate of
metal, having characters _excised_; warm it gently, and lay it upon the
surface of a clean mirror glass for a few minutes: then remove, allow to
cool, and _breathe_ upon the glass, when the outlines of the device will
be distinctly seen. A plate of polished Silver may be substituted for
the glass, and in place of developing the image by the breath, it may be
brought out by Mercurial vapour.
The second proposition of M. Moser is as follows:--"_Light_ acts on
bodies, and its influence may be tested by vapours that adhere to the
substance."--A plate of mirror glass is exposed in the Camera to a bright
and intense light; it is then removed and breathed upon, when an image
before invisible will be developed, the breath settling most strongly upon
the parts where the light has acted. A plate of polished Silver may be
used as before instead of glass, the vapour of Mercury or of water being
employed to develope the image. An _iodized Silver plate_ is still more
sensitive to the influence of the light, and receives a very sharp and
perfect impression under the action of the Mercury.
It seems therefore from these experiments and others not quoted, that
the surfaces of various bodies are capable of being modified by contact
with each other, or by contact with a ray of light, in such a way as to
impart an affinity for a vapour; and further, that many of the Salts of
Silver are in the list of substances admitting of such modification. But
it is also evident that the same condition of surface which causes a
vapour to settle in a peculiar manner also affects the behaviour of the
Silver Salt when treated with a reducing agent. Thus, if a clean glass
plate be touched in certain spots by the warm finger, the impression soon
disappears, but is again seen on breathing upon the glass; and if this
same plate be coated with a very delicate layer of Iodized Collodion and
passed through the Nitrate bath, the solution of Pyrogallic acid will
commonly produce a well-defined outline of the figure even before the
plate has been exposed to the light. This experiment, although it does
not invariably succeed, is nevertheless an instructive one, and shows the
necessity of cleaning the plates used in Photography with care. If there
be any irregularity in the manner in which the breath settles upon the
glass when it is breathed on, a condition of surface exists at that point
which will probably so modify the layer of Iodide of Silver, that the
action of the developing fluid will be in some way interfered with.
One more remarkable fact observed by M. Moser may be quoted. He finds that
the action of light upon the Daguerreotype plate is of an _alternating_
kind: it first gives an affinity for Mercury, and then removes it. "If
light acts on Iodide of Silver," he says, "it imparts to it the power of
condensing mercurial vapours; but if it acts beyond a certain time, it
then diminishes this power and at length takes it away altogether." This
is precisely in accordance with phenomena observed also in the Collodion
process, where the deposit of metallic Silver is sometime less marked than
usual if the plate has been exposed in the Camera beyond the proper period
of time.
A curious perversion of the developing process is occasionally met with,
in which on the application of the Pyrogallic Acid, the deposit of Silver
takes place upon the _shadows_ of the picture, and not upon the lights;
hence on viewing the image by transmitted light, the usual appearance is
reversed. This may perhaps be explained by an alternating action of the
light as above suggested.
A phenomenon at first sight even more remarkable has occurred, in
which, on developing the plate, _two_ images start out instead of
one. The secondary image in such a case is probably the remains of a
previous impression which, although apparently removed by washing, had
nevertheless modified the surface of the glass so as to affect the layer
of Iodide of Silver; and if the glass were _breathed_ upon before again
coating it with Collodion, there is every reason to suppose that the
outlines of the accidental image would be seen.[10]
[10] Since writing the above, the Author has perused with pleasure a paper
by Mr. Grove on the production of latent images by electricity, with a
mode of fixing them. In the experiments described, a plate of glass,
electrized in certain portions only, was breathed upon, or exposed to the
fumes of Hydrofluoric Acid. In either case the vapour settled exclusively
upon the non-electrical part of the glass, thus developing a latent image.
When the plate was first submitted to electrization, and then coated
with Iodide of Silver upon Collodion, and exposed to light,--solution of
Pyrogallic Acid produced a reduction of Silver only upon the parts of the
glass corresponding to those on which the breath settled in the previous
experiment; thus indicating that the electricity neutralized the effect of
light upon the sensitive Iodide of Silver.
CHAPTER IV.
ON FIXING THE PHOTOGRAPHIC IMAGE.
A sensitive layer of Chloride or Iodide of Silver on which an
image has been formed, either with or without the aid of a developing
agent, must pass through further treatment in order to render it
indestructible by diffused light.
It is true that the image itself is sufficiently permanent, and cannot be
said, in correct language, to need _fixing_; but the unchanged Silver Salt
which surrounds it, being still sensitive to light, tends to be decomposed
in its turn, and so the picture is lost. It is therefore necessary to
remove this salt by applying some chemical agent capable of dissolving
it. The list of solvents of Chloride and Iodide of Silver has been given
in Chapter II., but some are better adapted for fixing than others. In
order that any body may be employed with success as a fixing agent, it is
required not only that it should dissolve unchanged Chloride or Iodide of
Silver, but that it should produce no injurious effect upon the same salts
reduced by light.
This _solvent action upon the image_, as well as upon the parts which
surround it, is most liable to happen when the agency of light alone,
without a developer, has been employed. In that case the darkened surface,
not being reduced perfectly to the metallic state, remains soluble to a
certain extent in the fixing liquid.
CHEMISTRY OF THE VARIOUS FIXING AGENTS.
The following will be mentioned:--Ammonia--Alkaline Chlorides--Alkaline
Iodides--Alkaline Hyposulphite--Alkaline Cyanides.
AMMONIA.
The properties of the alkaline liquid "Ammonia" are given in Part III.
Ammonia dissolves Chloride of Silver readily, but not Iodide of Silver:
hence its use is necessarily confined to the paper proofs upon Chloride
of Silver. Even these however cannot advantageously be fixed in Ammonia
unless a deposit of Gold has been previously produced upon the surface
by a process of "toning," presently to be explained: a peculiar and
unpleasant red tint is always caused by Ammonia acting upon the darkened
material of a sun picture as it comes from the printing-frame: but this is
obviated by the employment of the Gold.
ALKALINE CHLORIDES, IODIDES, AND BROMIDES.
The Chlorides of Potassium, Ammonium, and Sodium possess the property of
dissolving a small portion of Chloride of Silver. In the act of solution
a double salt is formed; that is, a compound of Chloride of Sodium with
Chloride of Silver, which may be crystallized out by allowing the liquid
to evaporate spontaneously.
The earlier Photographers employed a saturated solution of common Salt for
fixing paper prints; but the fixing action of the Alkaline Chlorides is
slow and imperfect, and their use may now be said to be obsolete.
The Iodide and Bromide of Potassium have both been used as fixing agents.
They dissolve Iodide of Silver, forming with it a double salt in the
manner before described.
It is important to remark in the solution of the insoluble Silver Salts
by Alkaline Chlorides, Iodides, etc., that the amount dissolved is not
in proportion to the _quantity_ of the solvent, but to the degree of
concentration of its aqueous solution. This is not usual with solvents
which act by entering into chemical combination with the substance
dissolved. Commonly a given weight of the one salt dissolves a given
weight of the other, independent of the amount of water present. The
peculiarity in the case before us depends upon the fact that the double
salt formed is _decomposed_ by a large quantity of water. Hence it is a
_saturated_ solution of Chloride of Sodium which possesses the greatest
power of fixing paper prints; and with the Bromide or Iodide of Potassium
the same rule holds good--the stronger the solution the more Iodide of
Silver will be taken up. The addition of water produces milkiness and a
deposit of the silver Salt previously dissolved.
ALKALINE HYPOSULPHITES.
Hyposulphurous Acid is one of the Oxides of Sulphur. It is, as its name
implies, of an acid nature, and takes its place upon the list immediately
below Sulphurous Acid ("υρο," under).
The Hyposulphite of Soda commonly employed by Photographers is a neutral
combination of Hyposulphurous Acid and the alkali Soda. It is selected as
being more economical in preparation than any other Hyposulphite adapted
for fixing.
Hyposulphite of Soda occurs in the form of large translucent groups of
crystals, which include five atoms of water. These crystals are soluble
in water almost to any extent, the solution being attended with the
production of cold; they have a nauseous and bitter taste.
In the solution of Silver compounds by Hyposulphite of Soda a _double
decomposition_ always takes place; thus:--
Hyposulphite of Soda + Chloride of Silver
= Hyposulphite of Silver + Chloride of Sodium.
The Hyposulphite of Silver with an excess of Hyposulphite of Soda forms
a soluble double salt, which may be crystallized out by evaporating the
solution. It possesses an intensely sweet taste, and contains one atom of
Hyposulphite of Silver, chemically combined with two of Hyposulphite of
Soda. In addition to this there is a second double Salt, differing from
the first in being _very sparingly_ soluble in water. It is formed by
acting upon Chloride of Silver with a solution of Hyposulphite of Soda
already saturated, or nearly so, with Silver Salts; and contains single
atoms of each constituent.
The fact that the Silver contained in an ordinary fixing Bath is present
in the state of _Hyposulphite_ must be borne in mind, because this salt is
liable to undergo peculiar chemical changes, as will be better shown in
Chapter VIII.
Iodide of Silver is dissolved by Hyposulphite of Soda more slowly than
Chloride of Silver, and the amount eventually taken up is less. This is
explained as follows:-- During the solution of Iodide of Silver, _Iodide
of Sodium_ is formed, and this alkaline Iodide has a prejudicial effect
upon the continuance of the process. _Chloride_ of Sodium has not the same
action, neither has Bromide of Sodium, consequently the corresponding
Silver Salts dissolve to a greater extent than the Iodide.
ALKALINE CYANIDES.
The chemistry of Cyanogen is sketched in Part III.
The Cyanide of _Potassium_ is the salt most frequently employed in fixing.
It occurs in commerce in the form of fused lumps of considerable size. In
this state it is usually contaminated with a large percentage of Carbonate
of Potash, amounting in some cases to more than half its weight. By
boiling in proof Spirit the Cyanide may be extracted and crystallized, but
this operation is scarcely required as far as its use in Photography is
concerned.
Cyanide of Potassium absorbs moisture on exposure to the air. It is very
soluble in water, but the solution decomposes on keeping; changing in
colour and evolving the odour of _Prussic Acid_, which is a Cyanide of
Hydrogen. Cyanide of Potassium is highly poisonous, and must be used with
caution.
Solution of Cyanide of Potassium is a most energetic agent in dissolving
the insoluble Silver Salts: far more so, in proportion to the quantity
used, than the Hyposulphite of Soda. The Salts are in all cases converted
into Cyanides, and exist in the solution in the form of soluble double
Salts, which, unlike the double Iodides, are not affected by dilution with
water. Cyanide of Potassium is unadapted for fixing positive proofs upon
Chloride of Silver; and even when a developer has been used, unless the
solution is tolerably dilute, it is apt to attack the image and dissolve
it.
CHAPTER V.
ON THE NATURE AND PROPERTIES OF LIGHT.
The present Chapter is devoted to a discussion of the more remarkable
properties of Light; the object being to select certain prominent points,
and to state them as clearly as possible, referring, for information of a
more complete kind, to acknowledged works on the subject of Optics.
The Chapter will be divided into five Sections:--first, the compound
nature of Light; second, the laws of refraction of Light; third, the
construction of Lenses and of the Camera; fourth, the Photographic action
of coloured Light; fifth, on Binocular Vision and the Stereoscope.
SECTION I.
_The Compound Nature of Light._
The ideas entertained on the subject of Light, before the time of Sir
Isaac Newton, were vague and unsatisfactory. It was shown by that eminent
philosopher, that a ray of sunlight was not _homogeneous_, as had been
supposed, but consisted of several rays of vivid colours, united and
intermingled.
This fact may be demonstrated by throwing a pencil of Sunlight upon one
angle of a _prism_, and receiving the oblong image, so formed, upon a
white screen.
The space illuminated and coloured by a pencil of rays analyzed in this
way is called "the Solar Spectrum." The action of a prism in decomposing
white light will be more fully explained in the next Section. At present
we notice only that seven principal colours may be distinguished in the
Solar Spectrum, viz. red, orange, yellow, green, blue, indigo, and violet.
Sir David Brewster has made observations which lead him to suppose that
the _primary_ colours are in reality but three in number, viz. red,
yellow, and blue, and that the others are _compound_, being produced by
two or more of these overlapping each other; thus the red and yellow
spaces intermingled constitute _orange_; the yellow and blue spaces,
_green_.
[Illustration]
The composition of white light from the seven prismatic colours may be
roughly proved by painting them on the face of a wheel, and causing it to
rotate rapidly; this blends them together, and a sort of greyish-white
is the result. The white is imperfect, because the colours employed
cannot possibly be obtained of the proper tints or laid on in the exact
proportions.
The decomposition of light is effected in other ways besides that already
given:---
First, by _reflection_ form the surfaces of coloured bodies. All
substances throw off rays of light, which impinge upon the retina of the
eye and produce the phenomena of vision. Colour is caused by a _portion
only_, and not the whole, of the elementary rays, being projected in this
way. Surfaces termed _white_ reflect all the rays; coloured surfaces
absorb some and reflect others: thus _red_ substances reflect only red
rays, _yellow_ substances, yellow rays, etc, the ray which is reflected in
all cases deciding the colour of the substance.
Secondly, light may be decomposed by _transmission_ through media which
are transparent to certain rays, but opaque to others.
Ordinary transparent glass allows all the rays constituting white light
to pass; but by the addition of certain metallic oxides to it whilst in a
state of fusion, its properties are modified, and it becomes _coloured_.
Glass stained by Oxide of Cobalt is permeable only to blue rays. Oxide of
Silver imparts a pure yellow tint; Oxide of Gold or Suboxide of Copper a
ruby red, etc.
DIVISION OF THE ELEMENTARY RAYS OF WHITE LIGHT INTO LUMINOUS,
HEAT-PRODUCING, AND CHEMICAL. RAYS.
The agency of Light produces a variety of distinct effects upon the bodies
which surround us. These may be classed together as the properties of
light. They are of three kinds--the phenomena of colour and vision, of
heat, and of chemical action.
By resolving white light into its constituent rays, we find that these
properties are associated each one with certain of the elementary colours.
The _yellow_ is decidedly the most luminous ray. On examining the Solar
Spectrum, it is seen that the brightest part is that occupied by the
yellow, and that the light diminishes rapidly on either side. So again,
rooms glazed with yellow glass always appear abundantly illuminated,
whilst the effect of red or blue glass is dark and sombre. The yellow
colour therefore constitutes that portion of white light by which
surrounding objects are rendered visible; it is essentially the _visual_
ray.
The _heating properties_ of the sunlight reside principally in the red
ray, as is shown by the expansion of a mercurial thermometer placed in
that part of the spectrum.
The chemical action of light corresponds more to the indigo and violet
rays, and is wanting, as regards its influence upon Iodide of Silver, both
in the red and yellow. Strictly speaking however it cannot be localized in
either of the coloured spaces, as will be more fully shown in the Fourth
Section of this Chapter, to which the reader is referred.
SECTION II.
_The Refraction of Light._
A ray of light, in its passage through any transparent medium, travels in
a straight line as long as the density of the medium continues unchanged.
But if the density varies, becoming either greater or less, then the ray
is _refracted_, or bent out of the course which it originally pursued. The
degree to which the refraction or bending takes place depends upon the
nature of the new medium, and in particular upon its _density_ as compared
with that of the medium which the ray had previously traversed. Hence
Water refracts light more powerfully than Air, and Glass more so than
Water.
The following diagram illustrates the refraction of a ray of light.
[Illustration]
The dotted line is drawn perpendicularly to the surface, and it is
seen that the ray of light on entering is bent towards this line. On
emerging, on the other hand, it is bent to an equal extent _away from
the perpendicular_, so that it proceeds in a course parallel to, but not
coincident with, its original direction. If we suppose the new medium,
in place of being more dense than the old, to be _less dense_, then
the conditions are exactly reversed,--the ray is bent away from the
perpendicular on entering, and towards it on leaving.
It must be observed that the laws of refraction apply only to rays
of light which fall upon the medium _at an angle:_ if they enter
perpendicularly--in the direction of the dotted lines in the last
figure--they pass straight through without suffering refraction.
Notice also, that it is _at the surfaces of bodies_ that the deflecting
power acts. The ray is bent on entering, and bent again on leaving; but
whilst within the medium it continues in a straight line. Hence it is
evident that by variously modifying the surfaces of refractive media the
rays of light may be diverted almost at pleasure. This will be rendered
clear by a few simple diagrams.
In the figures given below, and in the following page, the dotted lines
represent perpendiculars to the surface at the point where the ray falls,
and it is seen that the usual law of bending _towards_ the perpendicular
on entering, and away from it on leaving the dense medium, is in each case
correctly observed.
[Illustration: Fig. 1.]
[Illustration: Fig. 2.]
Fig. 2, termed a prism, bends the ray permanently to one side; fig. 3,
consisting of two prisms placed base to base, causes rays before parallel
to meet in a point; and conversely, fig. 4, having prisms placed edge to
edge, diverts them further asunder.
[Illustration: Fig. 3.]
[Illustration: Fig. 4.]
_The various forms of Lenses._--The phenomena of the refraction of light
are seen in the case of curved surfaces in the same manner as with those
which are plane.
Glasses ground of a curvilinear form are termed _Lenses_. The following
are examples.
[Illustration: Fig. 1.]
[Illustration: Fig. 2.]
[Illustration: Fig. 3.]
Fig. 1 is a biconvex lens; fig. 2, a biconcave lens; and fig. 3, a
_meniscus_ lens.
As far as regards their refractive powers, such figures may be
represented, nearly, by others bound by straight lines, and thus it
becomes evident that a biconvex lens tends to condense rays of light to a
point, and a biconcave to scatter them. A meniscus combines both actions,
but the rays are eventually bent together, the convex curve of a meniscus
lens being always greater than the concave.
_The Foci of Lenses._--It has been shown that convex lenses tend to
condense rays of light and bring them together to a point. This point is
termed "the focus" of the Lens.
The following laws as regards the focus may be laid down:--
That rays of light which are pursuing a parallel course at the time they
enter the Lens are brought to a focus at a point nearer to the Lens
than diverging rays. The rays proceeding from very distant objects are
parallel; those from objects near at hand diverge. The sun's rays are
always parallel, and the divergence of the others becomes greater as the
distance from the Lens is less.
The focus of a Lens for parallel rays is termed the "principal focus,"
and is not subject to variation; this is the point referred to when the
_focal length_ of a Lens is spoken of. When the rays are not parallel, but
diverge from a point, that point is associated with the focus, and the two
are termed "conjugate foci."
[Illustration]
In the above diagram A is the principal focus, and B and C are conjugate
foci. Any object placed at B has its focus at G, and conversely when
placed at C it is in focus at B.
Therefore, although the principal focus of a Lens (as determined by the
degree of its convexity) is always the same, yet the focus for objects
near at hand varies, being longer as they are brought closer to the Lens.
_Formation of a Luminous Image by a Lens._--As the rays of light
proceeding from a point are brought to a focus by means of a Lens, so are
they when they proceed from an object, and in that case _an image of the
object_ is the result.
[Illustration]
The above figure illustrates this. The size of the image varies with the
distance of the arrow from the glass--being larger and formed at a point
further from the Lens as the object is brought nearer. The refracting
power of the Lens also influences the result--lenses of short focal
length, _i. e._ more convex, giving a smaller image.
In order that the course pursued by pencils of rays proceeding from an
object may be easily traced, the lines from the barb of the arrow in
the last figure are _dotted_. Observe that the object is necessarily
_inverted_, and also that those rays which traverse the central point of
the Lens, or the centre of the _axis_, as it is termed, are not bent away,
but pursue a course either coincident with, or parallel to, the original,
as in the case of refracting media with parallel surfaces.
SECTION III.
The Photographic Camera.
The Photographic Camera is in its essential nature an extremely simple
instrument. It consists merely of a _dark chamber_, having an aperture
in front in which a Lens is inserted. The accompanying figure shows the
simplest form of Camera.
[Illustration]
The body is represented as consisting of two portions which slide within
each other; but the same object of lengthening or shortening the focal
distance may be attained by making the Lens itself movable. A luminous
image of any object placed in front of the Camera is formed by means of
the Lens, and received upon a surface of ground glass at the back part
of the instrument. When the Camera is required for use, the object is
_focussed_ upon the ground glass, which is then removed, and a slide
containing the sensitive layer inserted in its place.
The luminous image, as formed upon the ground glass, is termed the
"Field" of the Camera; it is spoken of as being flat or curved, sharp
or indistinct, etc. These and other peculiarities which depend upon the
construction of the Lens will now be explained.
_Chromatic Aberration of Lenses._--The outside of a biconvex lens is
strictly comparable with the sharp edge of a _prism_, and therefore
necessarily produces decomposition in the white light which passes through
it.
The action of a prism in separating white light into its constituent rays
may be simply explained;--all the coloured rays are refrangible, but not
to the same extent. The indigo and violet are more so than the yellow and
red, and consequently they are separated from them, and occupy a higher
position in the Spectrum. (See the diagram at p. 47.)
A little reflection will show that in consequence of this unequal
refrangibility of the coloured rays, white light must invariably be
decomposed on entering any dense medium. This is indeed the case; but if
the surfaces of the medium _are parallel to each other_ the effect is
not seen, because the rays recombine on their emergence, being bent to
the same extent in the opposite direction. Hence light is transmitted
colourless through an ordinary pane of glass, but yields the tints of the
Spectrum in its passage through a prism or a lens, where the two surfaces
are inclined to each other at an acute angle.
Chromatic aberration is corrected by combining two lenses cut from
varieties of glass which differ in their power of separating the coloured
rays. These are the dense flint-glass containing Oxide of Lead, and the
light crown-glass. Of the two lenses, the one is _biconvex_, and the
other _biconcave_; so that when fitted together they produce a compound
Achromatic lens of a meniscus form, thus:--
[Illustration]
The first Lens in this figure is the flint- and the second the
crown-glass. Of the two the biconvex is the most powerful, so as to
overcome the other, and produce a total of refraction to the required
extent. Each of the Lenses produces a spectrum of a different length;
and the effect of passing the rays through both, is, by overlapping the
coloured spaces, to unite the complementary tints, and to form again white
light.
Spherical Aberration of Lenses.--The field of a Camera is not often
equally sharp and distinct at every part. If the centre be rendered clear
and well defined, the outside is misty; whilst, by slightly altering the
position of the ground glass, so as to define the outside portion sharply,
the centre is thrown out of focus. Opticians express this by saying that
there is a want of proper flatness of field; two causes may be mentioned
as concurring to produce it.
The first is "spherical aberration," by which is meant the property
possessed by Lenses which are segments of spheres, of refracting rays
of light unequally at different parts of their surfaces. The following
diagram shows this:--
[Illustration]
Observe that the dotted lines which fall upon the circumference of the
Lens are brought to a focus at a point nearer to the Lens than those
passing through the centre; in other words, the outside of the Lens
refracts light the most powerfully. This causes a degree of confusion and
indistinctness in the image, from various rays crossing, and interfering
with, each other.
Spherical aberration may be avoided by increasing the convexity of the
centre part of the Lens, so as to add to its refracting power at that
particular point. The surface is then no longer a segment of a sphere, but
of an ellipse, and refracts light more equally. The difficulty of grinding
Lenses to an elliptical form however is so great, that the spherical Lens
is still used, the aberration being corrected in other ways.
A second cause interfering with the distinctness of the outer portions
of the image in the Camera is the obliquity of some rays proceeding from
the object; in consequence of which the image has a curved form, with the
concavity inwards, as may be seen by referring to the figure given at page
53. The following diagram is meant to explain curvature of the image.
The centre line running at right angles to the general direction of the
Lens is the axis; an imaginary line, on which the Lens may be said to
rotate as a wheel turns on its axle. The lines A A represent rays of light
falling parallel to the axis; and the dotted lines, others which have
an oblique direction; B and C show the points at which the two foci are
formed. Observe that these points, although equidistant from the centre
of the Lens, do not fall in the same vertical plane, and therefore they
cannot both be received distinct upon the ground glass of the Camera,
which would occupy the position of the perpendicular double line in the
diagram. Hence it is that with most lenses, when the centre of the field
has been focussed, the glass must be shifted forwards a little to define
the outside sharply.
[Illustration]
_The Use of Stops in Lenses._--Curvature of the image and indistinctness
of outline from spherical aberration are both remedied to a great extent
by fixing in front of the Lens a diaphragm having a small central
aperture. The diagram gives a sectional view of a Lens with a "stop"
attached; the exact position it should occupy with reference to the Lens
is a point of importance, and influences the flatness of the field.
[Illustration]
By using a diaphragm the quantity of light admitted into the Camera
is diminished in proportion to the size of the aperture. The image is
therefore less brilliant, and a longer exposure of the sensitive plate is
required. In other respects however the result is improved; the spherical
aberration is lessened by cutting off the outside of the Lens, and a
portion of the oblique rays being intercepted, the focus of the remainder
is lengthened out, and the image is rendered flatter, and improved in
distinctness. Hence also, when a small stop is affixed to a Lens, a
variety of objects, situated at different distances, are all in focus at
once; whereas, with the full aperture of the Lens, objects near at hand
cannot be rendered distinct upon the ground glass at the same time with
distant objects, or _vice versâ_.
_The Double or Portrait Combination of Achromatic Lenses._--The brightness
of illumination of an image formed by a Lens is in proportion to the
diameter of the Lens, that is, to the size of the aperture by which the
Light is admitted. The _clearness or distinctness of outline_ however is
independent of this, being improved by using a stop, which lessens the
diameter.
The Portrait combination of Lenses is constructed to ensure rapidity of
action by admitting a large volume of light. The following diagram gives a
sectional view.
[Illustration]
In this combination the front Lens is an Achromatic plano-convex, with,
the convex side turned toward the object; and the second, which takes up
the rays and refracts them further, is a compound Biconvex Lens; there are
therefore in all four distinct glasses concerned in forming the image,
which may appear at first to be an unnecessarily complex arrangement. It
is found however that a good result cannot be secured by using a single
Lens, when a "stop" is inadmissible. By combining two glasses of different
curves, the aberrations of one correct those of the other to a great
extent, and the field is both flatter and more distinct than in the case
of an Achromatic Meniscus employed without a diaphragm.
The manufacture of Portrait Lenses is a point of great difficulty, the
glasses requiring to be ground with extreme care, in order to avoid
_distortion_ of the image: hence the most rapid Portrait Lenses, having
large aperture and short focus, are often useless unless purchased of a
good maker.
_The Variation between the Visual and Actinic Foci in Lenses._--The same
causes which produce chromatic aberration in a Lens, tend also to separate
the chemical from the visual focus.
The violet and indigo rays are more strongly bent in than the yellow, and
still more than the red; consequently the focus for each of those colours
is at a different point. The following diagram shows this.
[Illustration]
V represents the focus of the violet ray, Y of the yellow, and E of the
red.
Hence, as the chemical action corresponds more to the violet, the most
marked actinic effect would be produced at V. The luminous portion of the
spectrum however is _the yellow_, consequently the visual focus is at Y.
Photographers have long recognized this point; and therefore, with
ordinary Lenses, not corrected for colour, rules are laid down as to the
exact distance which the sensitive plate should be shifted away from the
visual focus in order to obtain the greatest amount of distinctness of
outline in the image impressed by chemical action.
These rules do not apply to the Achromatic Lenses recently described. The
coloured rays being in that case bent together again and reunited, the two
foci also nearly correspond. By a little further correction to a point
higher in the Spectrum, they are made to do so perfectly.
SECTION IV.
_On the Photographic Action of Coloured Light._
It has already been mentioned in the First Section of this Chapter that
certain of the elementary colours of white light, viz. the violet and
indigo, are peculiarly active in decomposing the Photographic Salts of
Silver; but there are some points of importance relating to the same
subject which require a further notice.
The term "actinism" (Gr. ἁκτἱς, a ray or flash) has been proposed as
convenient to designate the property possessed by light of producing
chemical change; the rays to which the effect is especially due being
known as actinic rays.
If the pure Solar Spectrum formed by prismatic analysis in the manner
represented at page 47 be allowed to impinge upon a prepared sensitive
surface of Iodide of Silver, the latent image being subsequently developed
by a reducing agent, the effect produced will be something similar to that
represented in the following diagram:--
[Illustration: Fig. 1.]
[Illustration: Fig. 2.]
Fig. 1 shows the visible spectrum as it appears to the eye; the brightest
part being in the yellow space, and the light gradually shading off
until it ceases to be seen. Fig. 2 represents the chemical effect
produced by throwing the Spectrum upon Iodide of Silver. Observe that
the darkening characteristic of chemical action is most evident in the
upper spaces, where the light is feeble, and is altogether absent at the
point corresponding to the bright yellow spot of the visible spectrum.
The actinic and luminous spectra are therefore totally distinct from each
other, and the word "Photography," which signifies the process of taking
pictures by light, is in reality inaccurate.
To those who have not the opportunity of working with the Solar Spectrum,
the following experiments will be useful in illustrating the Photographic
value of coloured light.
_Experiment I._--Take a sheet of sensitive paper prepared with Chloride
of Silver, and lay upon it strips of blue, yellow, and red glass. On
exposure to the sun's rays for a few minutes, the part beneath the blue
glass darkens rapidly, whilst that covered by the red and yellow glass is
perfectly protected. This result is the more striking from the extreme
_transparency_ of the yellow glass, giving the idea that the Chloride
would certainly be blackened first at that point. On the other hand, the
blue glass appears very dark, and effectually conceals the tissue of the
paper from view.
_Experiment II._--Select a vase of flowers of different shades of scarlet,
blue, and yellow, and make a Photographic copy of them, by development,
upon Iodide of Silver. The blue tints will be found to act most violently
upon the sensitive compound, whilst the reds and yellows are scarcely
visible; were it not that it is difficult to procure in nature pure and
homogeneous tints, free from admixture with other colours, they would make
no impression whatever upon the plate.
In exemplifying further the importance of distinguishing between visual
and actinic rays of light, we may observe that if the two were in all
respects the same. Photography must cease to exist as an Art. It would
be impossible to make use of the more sensitive chemical preparations
from the difficulties which would attend the previous preparation and
subsequent development of the plates. These operations are now conducted
in what is termed a dark room; but it is dark only in a _Photographic_
sense, being illuminated by means of yellow light, which, whilst it
enables the operator easily to watch the progress of the work, produces
no injurious effect upon the sensitive surfaces. If the windows of the
room were glazed with _blue_ in place of yellow glass, then it would
be strictly a "dark room," but one altogether unfitted for the purpose
intended.
Another point connected with the same subject and worthy of note
is--the extent to which the sensibility of the Photographic compounds is
influenced by atmospheric conditions not visibly interfering with the
_brightness_ of the light. It is natural to suppose that those days on
which the sun's rays are the most powerful would be the best for rapid
impression, but such is not by any means the case. If the light is at all
of a yellow cast, however bright it may be, its actinic power will be
small.
It will also be often observed in working towards the evening, that a
sudden diminution of sensibility in the plates begins to be perceptible at
a time when but little difference can be detected in the brilliancy of the
light; the setting sun has sunk behind a golden cloud, and all chemical
action is soon at an end.
In the same manner is explained the difficulty of obtaining Photographs
in the glowing light of tropical climates; the superiority Of the early
months of spring over those of the midsummer; of the morning sun to that
of the afternoon, etc. April and May are usually considered the best
months for rapid impression in this country; but the light continues good
until the end of July. In August and September a longer exposure of the
plates will be required.
THE SUPERIOR SENSIBILITY OF BROMIDE OF SILVER TO COLOURED LIGHT.
In copying the Solar Spectrum alternately upon a surface of Iodide and
Bromide of Silver, we notice a difference in the Photographic properties
of these two salts. The latter is affected more extensively, to a point
lower in the spectrum, than the former. In the case of the Iodide of
Silver, the action ceases in the Blue space; but with the Bromide it
reaches to the Green. This is shown in the following diagrams, which are
drawn from the observations of Mr. Crookes ('Photographic Journal,' vol.
i. p. 100):--
[Illustration: Fig. 1. Fig. 2. Fig. 3.]
Fig. 1 represents the chemical spectrum on Bromide of Silver; fig. 2, the
same upon Iodide of Silver; and fig. 3, the visible spectrum.
It might perhaps be supposed that the superior sensibility of the Bromide
of Silver to green rays of light would render that salt useful to the
Photographer in copying landscape scenery; and indeed it is the opinion of
many that, in the _Calotype_ paper process, the dark colour of foliage is
better rendered by a mixture of Bromide and Iodide of Silver than by the
latter salt alone. This however cannot depend upon the greater sensibility
of the Bromide to coloured light, as may easily be proved.--
The diagrams given above are shaded to represent nearly, the relative
intensity of the chemical action exerted by the rays at different points
of the spectrum; and on referring to them it will be seen that the
maximum point of blackness is in the indigo and violet space, the action
being more feeble in the blue space lower down; there are also highly
refrangible rays extending upwards far beyond the visible colours, and
these invisible rays are actively concerned in the formation of the image.
It is evident therefore that the amount of effect produced by a pure
green, or even a light blue tint, upon a surface of Bromide of Silver is
very small as compared with that of an indigo or violet; and hence, as in
copying natural objects radiations of all kinds are present at the same
time, the green tints have not time to act before the image is impressed
by the more refrangible rays.
Sir John Herschel proposed to render coloured light more available in
Photography by separating the actinic rays of high refrangibility, and
working only with those which correspond to the blue and green spaces
in the spectrum. This may be done by placing in front of the Camera a
vertical glass trough containing a solution of Sulphate of Quinine.
Professor Stokes has shown that this liquid possesses curious properties.
In transmitting rays of light it _modifies_ them so that they emerge _of
lower refrangibility_, and incapable of producing the same actinic effect.
Sulphate of Quinine is, if we may use the term, _opaque_ to all actinic
rays higher than the blue-coloured space. The proposition of Sir John
Herschel above referred to was therefore to employ a bath of Sulphate of
Quinine, and having eliminated the actinic rays of high refrangibility, to
work upon Bromide of Silver with those corresponding to the lower-coloured
spaces. In this way he conceived that a more natural effect might be
obtained.
If Photographic compounds should be discovered of greater sensibility
than any we at present possess, the use of the Quinine bath will perhaps
be adopted; but at present we trust to the superior intensity of the
invisible rays for the formation of the image, and hence the employment of
Bromide of Silver is less strongly indicated.
These remarks apply to Photographs taken by sunlight. Mr. Crookes states
that in working with artificial light, such as gas or camphine, the case
is different. Actinic rays of high refrangibility are comparatively
wanting in gas-light, the great bulk of the Photographic rays beings found
to lie within the limits of the visible spectrum, and consequently acting
more energetically upon Bromide than on Iodide of Silver.
_Explanation of the mode in which Coloured Objects impress the Sensitive
Film._--The fact of which we have been speaking, viz. that the natural
colours are not always correctly represented in photography, is often
urged in depreciation of the art,--"when lights, are represented by
shadows," it is said, "how can a truthful picture be expected?" The
insensitiveness of Iodide of Silver to the colours occupying the lower
portion of the spectrum would indeed present an insuperable difficulty _if
the tints of Nature were pure and homogeneous:_ such however is not the
case. Even the most sombre colours are accompanied by scattered rays of
white light in quantity amply sufficient to affect the sensitive film.
This is especially seen when the coloured body _possesses a good
reflecting surface;_ and hence some varieties of foliage, as for instance
the Ivy, with its smooth and polished leaf, are more easily photographed
than others. So again with regard to drapery in the department of
portraiture--it is necessary to attend not only to the colour, but also to
the material of which it is composed. Silks and satins are favourable, as
reflecting much light, whilst velvets and coarse stuffs of all kinds, if
at all dark, produce very little effect upon the sensitive film.
SECTION V.
_On Binocular Vision and the Stereoscope._
An object is said to be "stereoscopic" (στρεοϛ solid, and σκοπεω, I see)
when it stands out in relief, and gives to the eye the impression of
solidity.
This subject was first explained by Professor Wheatstone in a memoir on
binocular vision, published in the 'Philosophical Transactions' for 1838;
in which he shows that solid bodies project different perspective figures
upon each retina, and that the illusion of solidity may be artificially
produced by means of the "Stereoscope."
The phenomena of binocular vision may be simply sketched as follows:--If a
cube, or a small box of an oblong form, be placed at a short distance in
front of the observer, and viewed attentively with the right and left eye
separately and in succession, it will be found that the figure perceived
in the two cases is different; that each eye sees more of one side of the
box, and less of the other; and that in neither instance is the effect
exactly the same as that given by the two eyes employed conjointly.
A silver pencil-case, or a pen-holder, may be used to illustrate the same
fact. It should be held at about six or eight inches distant from the root
of the nose, and quite at right angles to the face, so that the length of
the pencil is concealed by the point. Then, whilst it remains fixed in
this position, the left and right eye are to be alternately closed: in
each case a portion of the opposite side of the pencil will be rendered
visible.
[Illustration: Fig. 1. Fig. 2.]
The preceding diagrams exhibit the appearance of a bust as seen by each
eye successively.
Observe that the second figure, which represents the impression received
by the right eye, is more of a full face than fig. 1, which, being viewed
from a point removed a little to the left, partakes of the character of a
profile.
The human eyes are placed about 2-1/2 inches, or from that to 2-5/8
inches, asunder; hence it follows that, the points of sight being
separated, a _dissimilar_ image of a solid object is formed by each eye.
We do not however see two images, but a single one, which is stereoscopic.
In looking at a picture painted on a flat surface the case is different:
the eyes, as before, form two images, but these images are in every
respect similar; consequently the impression of solidity is wanting. A
single picture, therefore, cannot be made to appear stereoscopic. To
convey the illusion _two_ pictures must be employed, the one being a right
and the other a left perspective projection of the object. The pictures
must also be so arranged, that each is presented to its own eye, and that
the two appear to proceed from the same spot.
The reflecting stereoscope, employed to effect this, forms _luminous
images_ of the binocular pictures, and throws these images together, so
that, on looking into the instrument, only a single image is seen, in
a central position. It should, however, be understood, that no optical
arrangement of any kind is indispensably required, since it is quite
possible, with a little effort, to combine the two images by the unaided
organs of vision. The following diagram will make this obvious:--
[Illustration]
The circles A and B represent two wafers, which are stuck on paper at
a distance of about three inches from each other. They are then viewed
by _squinting_ strongly, or turning the eyes inwards towards the nose,
until the right eye looks at the left wafer, and the left eye at the right
wafer. Each wafer will then appear to become double, four images being
seen, the two central of which will gradually approach each other until
they coalesce. Stereoscopic pictures, properly arranged, may be examined
in the same manner; and it will be found that the resultant solid image is
formed midway, at a point where two lines, drawn across from the eyes to
the pictures, cut one another. The experiment here mentioned is sometimes
a painful one, and cannot easily be made if the eyes are not of equal
strength; but it will serve to show that the essential principle resides
in the binocular representation of the object, and not in the instrument
employed to view it.
In Mr. Wheatstone's reflecting Stereoscope _mirrors_ are used. The
principle of the instrument is as follows:-- objects placed in front of
a mirror have their reflected images apparently _behind_ the mirror. By
arranging two mirrors at a certain inclination to each other, the images
of the double picture may be made to approach until they coalesce, and the
eye perceives a single one only. The following diagram will explain this.
[Illustration]
The rays proceeding from the star on either side pass in the direction of
the arrows, being thrown off from the mirror (represented by the thick
black line) and entering the eyes at R and L. The reflected images appear
behind the mirror, uniting at the point A.
The reflecting Stereoscope is adapted principally for viewing large
pictures. It is a very perfect instrument, and admits of a variety of
adjustments, by which the apparent size and distance of the Stereoscopic
image may be varied almost at pleasure.
The "lenticular" Stereoscope of Sir David Brewster is a more portable form
of apparatus. A sectional view is given in the diagram.
[Illustration]
The brass tubes to which the eyes of the observer are applied contain
each a semi-lens, formed by dividing a common lens through the centre and
cutting each half into a circular form (fig. 1 in the following page). The
half-lens viewed in section (fig. 2) is therefore of a prismatic shape,
and when placed with its sharp edge as in the diagram above, alters the
direction of the rays of light proceeding from the picture, bending them
outwards or away from the centre, so that in accordance with well-known
optical laws they appear to come in the direction of the dotted lines in
the diagram (in the last page), and the two images coalesce at their point
of junction. In the instrument as it is often sold, one of the lenses is
made movable, and by turning it round with the finger and thumb it will be
seen that the positions of the images may be shifted at pleasure.
Fig. 1.
[Illustration]
Fig. 2.
_Rules for taking Binocular Photographs._--In viewing very distant objects
with the eyes, the images formed on the retinæ are not sufficiently
dissimilar to produce a very Stereoscopic effect; hence it is often
required, in taking binocular pictures, to separate the Cameras more
widely than the two eyes are separated, in order to give a sufficient
appearance of relief. Mr. Wheatstone's original directions were, to allow
about one foot of separation for each twenty-five feet of distance, but
considerable latitude may be permitted.
If the Cameras be not placed far enough apart, the dimensions of the
stereoscopic image from before backwards will be too small,--statues
looking like bas-reliefs, and the circular trunks of trees appearing oval,
with the long diameter transverse. On the other hand, when the separation
is too wide, the reverse obtains,--objects for instance which are square,
assuming an oblong shape pointing towards the observer.
To understand the cause of this, the following law in optics should be
studied:--"The distance of objects is estimated by the extent to which
the axes of the eyes must be converged to view them." If we have to turn
our eyes strongly inwards, we judge the object to be near; but if the eyes
remain nearly parallel, we suppose it to be distant.
[Illustration]
The above figures represent six-sided truncated pyramids, each with
its apex towards the observer, the centres of the two smaller interior
hexagons being more widely separated than those of the larger exterior
ones. By converging the eyes upon them so as to unite the central images
in the manner represented in page 68 a greater amount of convergence
will be required to bring together the two summits than the bases, and
hence the summits will appear the nearest to the eye; that is to say,
the resultant central figure will acquire the additional dimension of
_height_, and appear as a solid cone, standing perpendicularly upon its
base: further, the more widely the summits are separated in relation to
the bases, the taller will the cone be, although a greater effort will be
required to coalesce the figures.
Binocular Photographs taken with too much separation of the Cameras, are
distorted from a similar cause,--so strong a convergence being required to
unite them that certain parts of the picture appear to approach near to
the eye; and the depth of the solid image is increased.
This effect is most observable when the picture embraces a variety of
objects, situated in different planes. In the case of views which are
quite distant, no near objects being admitted, the Cameras may be placed
with especial reference to them, even as far as twelve feet apart, without
producing distortion.
It is sometimes observable, in looking at Stereoscopic pictures, that
they convey an erroneous impression of the real size and distance of the
object. For instance, in using the large reflecting Stereoscope, if,
when the adjustments have been made and the images properly united, the
two pictures be moved slowly forward, the eyes remaining fixed upon the
mirrors, the Stereoscopic image will gradually change its character,
the various objects it embraces appearing to become diminished in size,
and approaching near to the observer: whilst if the pictures be pushed
_backwards_, the image will enlarge and recede to a distance. So, again,
if an ordinary slide for the lenticular Stereoscope be divided in the
centre, and, looking into the instrument until the images coalesce, the
two halves be slowly separated from each other, the solid picture will
seem to become larger and to recede from the eye.
It is easy to understand the cause of this. When the pictures in the
reflecting Stereoscope are moved _forwards_, the convergence of the optic
axes is increased: the image therefore appears _nearer_, in accordance
with the last-mentioned law. But to convey the impression of nearness
is equivalent to an apparent diminution in size, for we judge of the
dimensions of a body very much in relation to its supposed distance. Of
two figures, for instance, appearing of the same height, one known to
be a hundred yards off might be considered colossal, whilst the other,
obviously near at hand, would be viewed as a statuette.
These facts, with others not mentioned, are of great interest and
importance, but their further consideration does not fall within the
bounds originally prescribed to us. The practical details of Stereoscopic
Photography have been arranged in a distinct Section, and will be found
included in the Second Part of the Work.[11]
[11] For a more full and detailed explanation of the Stereoscopic
phenomena, see an abstract of Professor Tyndall's lectures in the third
volume of the 'Photographic Journal.'
CHAPTER VI.
THE PHOTOGRAPHIC PROPERTIES OF IODIDE OF SILVER UPON COLLODION.
In the preceding part of this Work the physical and chemical properties of
Chloride and Iodide of Silver have been described, with the changes which
they experience by the action of Light. Nothing however has been said of
the surface used to support the Iodide of Silver, and to expose it in a
finely divided state to the influence of the actinic radiations. This
omission will now be supplied, and the use of Collodion will engage our
attention.
The sensibility of Iodide of Silver upon Collodion is greatly superior to
that of the same salt employed in conjunction with any other vehicle at
present known. Hence the Collodio-Iodide film will supersede the paper
and Albumen processes in all cases where objects liable to move are to be
copied. The causes of this superior sensitiveness, as far as ascertained,
may be referred to the state of _loose coagulation_ of a Collodion film
and other particulars presently to be noticed. It must however be allowed
that there are yet some points affecting the sensitiveness of Iodide of
Silver, both mechanical and chemical, of the exact nature of which we are
ignorant.
The present Chapter may be divided into four Sections:--the nature of
Collodion; the chemistry of the Nitrate Bath; the causes affecting the
formation and development of the Image upon Collodion; the various
irregularities in the development of the Image.
SECTION I.
_Collodion._
Collodion (so named from the Greek word κολλἁω, to _stick_) is a
glutinous, transparent fluid, procured, as generally said, by dissolving
Gun-Cotton in Ether. It was originally used for surgical purposes only,
being smeared over wounds and raw surfaces, to preserve them from
contact with the air by the tough film which it leaves on evaporation.
Photographers employ it to support a delicate film of Iodide of Silver
upon the surface of a smooth glass plate.
Two elements enter into the composition of Collodion: first, the
Gun-Cotton; second, the fluids used to dissolve it. Each of these will be
treated in succession.
CHEMISTRY OF PYROXYLINE.
Gun-Cotton or _Pyroxyline_ is Cotton or Paper which has been altered in
composition and properties by treatment with strong acids.
Both Cotton and Paper are, chemically, the same. They consist of fibres
which are found on analysis to have a constant composition, containing
three elementary bodies, Carbon, Hydrogen, and Oxygen, united together
in fixed proportions. To this combination the term _Lignine_ or
_Cellulose_[12] has been applied.
[12] Lignine and Cellulose are not precisely identical substances. The
latter is the material composing the cell-wall; the former, the contained
matter in the cell.
Cellulose is a definite chemical compound, in the same sense as Starch or
Sugar, and consequently, when treated with various reagents, it exhibits
properties peculiar to itself. It is insoluble in most liquids, such as
Water, Alcohol, Ether, etc., and also in dilute acids; but when acted
upon by Nitric Acid of a certain strength it liquefies and dissolves.
It has been already shown (p. 12) that when a body dissolves in Nitric
Acid the solution is not usually of the same nature as an aqueous
solution; and so in this case--the Nitric Acid imparts Oxygen first to the
Cotton, and afterwards dissolves it.