began to be supplanted by a rival method, devised by an Englishman, Mr.
Fox Talbot, who had published his process six months before that of
Daguerre was given to the world, and who, therefore, was unacquainted
with the details of the latter. The first of Mr. Talbot’s publications
contained only an improved mode of preparing a sensitive paper for
copying prints, by applying them to it and causing the light to pass
through the paper of the print, so that the parts of the sensitive paper
protected by the opaque black lines were not acted upon by the light.
The paper was first dipped in a solution of chloride of sodium, and then
in one of nitrate of silver, the result being the formation in the pores
of the paper of chloride of silver, a substance much more quickly
affected by light than the nitrate of silver used by Davy and Wedgwood.
The impression so obtained was a _negative_, that is, the lights and
shades of the original were reversed; but when this negative was again
copied by the same process, it produced a perfect copy of the original
print, for the lights and shades were of course reversed from those in
the negative proof. Thus from one negative any number of positive or
natural copies could be produced; and this point in Mr. Talbot’s
invention is one great feature of photography as now practised. In 1841,
Mr. Talbot obtained a patent for a process he called the _Calotype_, but
which, in his honour, has since been known as the Talbotype. A sheet of
paper is soaked, first in a solution of nitrate of silver, and then in
one of iodide of potassium, by which it becomes covered with iodide of
silver; it may then be dried. It is prepared for the camera by brushing
it over with a solution of gallic acid containing a little nitrate of
silver. By this last process its sensitiveness is greatly increased, and
an exposure in the camera for a few seconds, or minutes, according to
the power of the light, suffices to impress the paper with a latent or
invisible image, which reveals itself when the paper is treated with a
fresh portion of the gallic acid mixture. The Talbotype is the
foundation of the methods of photography now in general use; but, before
we describe these, it may be proper to mention some other substances
which have been found sensitive to light, and to discuss the nature of
the invisible images which are first produced in these processes.
The _art_ of photography has outstripped the _science_—in other words,
the nature and laws of the chemical actions by which its beautiful
effects are produced are not yet clearly understood, and some quite
recent discoveries seem to show that we have yet much to learn before a
complete theory of the chemical action of light can be proposed. Some
results which have been established may be mentioned, as they show those
curious effects of light to be more general than would be supposed from
a description of photographic processes dependent on silver salts only.
It has been found that certain acids, certain salts, and certain
compounds containing only two elements—of which one is a metal—have a
tendency to split up, or resolve themselves into their several
constituents, when exposed to the action of light. On the other hand,
chlorine, bromine, and iodine exhibit, under the same conditions, an
exalted affinity for the hydrogen of organic matters. These tendencies
concur when the compounds above referred to are associated with organic
materials, as in photography. Solution of nitrate of silver is blackened
when it is exposed to light on a piece of paper which has been dipped
into the solution; but a piece of white unglazed porcelain similarly
treated shows no change. A solution of nitrate of uranium in pure water
is not changed by light; but a solution of the same salt in alcohol
becomes green, and deposits oxide of uranium. The reducing action of the
light is insufficient of itself to accomplish the decomposition of the
salt in the first case; but the presence of the organic matter
determines this decomposition in the second case. Bichromate of
potassium is by itself not easily decomposed by light; but when it is
mixed with sugar, starch, gum, or gelatine, the sunbeams readily reduce
it. It is remarkable that the gelatine, gum, or starch becomes insoluble
by thus taking up oxygen, and the gelatine loses its property of
swelling up in water. We shall presently see the advantages which have
been drawn from these circumstances.
It is not necessary that the light should act upon both the organic
substance and the oxidizing substance at the same time. If paper
impregnated with iodide of silver and gallic acid be placed in the
camera, the image soon appears; but if, as in the Talbotype, the iodide
of silver only be acted upon by the light, no image is perceptible on
withdrawing the paper from the camera. The action of the light has
nevertheless imparted to the silver salt a tendency to reduction; for
when the paper is afterwards dipped into a solution of gallic acid, the
image immediately appears. In order to distinguish these two actions,
the substance which receives and preserves the latent impression from
the light is called the _sensitive_ substance, and that which reveals
the latent image is termed the _developing_ substance. A considerable
number of substances having this relation to each other have been
observed, and the following table of instances—cited by Niepce de
Saint-Victor, the nephew of the original inventor—will give some idea of
their variety:
┌───────────────────┬────────────────────────┬────────────────────────┐
│ Sensitive │ Developing Substance. │ Results. │
│ Substances in the │ │ │
│ paper exposed to │ │ │
│ the action of the │ │ │
│ Light. │ │ │
├───────────────────┼────────────────────────┼────────────────────────┤
│None, _i.e._, plain│A salt of silver │Black image. │
│ paper. │ │ │
│ │ │ │
│Nitrate of silver, │Gallic acid, or sulphate│Black image. │
│ or iodide of │ of iron. │ │
│ silver. │ │ │
│ │ │ │
│ │{ Water │By prolonged action of │
│ │ │ light, a grey image of│
│ │ │ protoxide of uranium; │
│ │ │ the image disappears │
│ │ │ when paper is kept in │
│ │ │ the dark, but shows │
│ │ │ itself again in the │
│ │ │ light. │
│Nitrate of uranium.│{ │ │
│ │{ Red prussiate of │Intensely red positive │
│ │ potash │ image; becomes blue by│
│ │ │ sulphate of iron. │
│ │ │ │
│Nitrate of uranium │Nitrate of silver or │Unchangeable │
│ and tartaric │ chloride of gold. │ images—resembling │
│ acid. │ │ those of ordinary │
│ │ │ photographs. │
│ │ │ │
│Chloride of gold. │Nitrate of uranium, │. . . . . . │
│ │ sulphate of iron, │ │
│ │ sulphate of copper, │ │
│ │ bichloride of mercury,│ │
│ │ salt of tin. │ │
│ │ │ │
│ │{ Sulphate of iron │Blue-black image. │
│Gallic acid. │{ │ │
│ │{ Red prussiate of │Blue image. │
│ │ potash │ │
│ │ │ │
│Red prussiate of │Water, bichloride of │Blue image, hastened by │
│ potash. │ mercury, gallic acid, │ acids and by heat. │
│ │ salt of silver, salt │ │
│ │ of cobalt. │ │
│ │ │ │
│Bichloride of │Protochloride of tin, │. . . . . . │
│ mercury. │ soda, potash, sulphide│ │
│ │ of sodium. │ │
│ │ │ │
│Chromic acid, or │Salts of silver │Purple-red positive │
│ bichromate of │ │ image. │
│ potash. │ │ │
│ │ │ │
│ │{ Blue litmus │Red image. │
│Starch. │{ Iodide of potassium │Reddish brown image. │
│ │{ White indigo │Blue positive image. │
│ │{ Campeachy wood │Red positive image. │
└───────────────────┴────────────────────────┴────────────────────────┘
These are only a few of the instances in which actions of this kind have
been observed. It is remarkable that the order of the first two columns
in this table may be inverted without changing the result. Thus, instead
of exposing iodide of silver to the light and developing the image with
gallic acid, one may expose a paper saturated with gallic acid solution,
and develop with iodide of potassium and nitrate of silver. The first
reaction noted in the table deserves some remark: it is not peculiar to
paper, but is common to most organic materials, such as albumen,
collodion starch, fabrics, and indeed to organic matters in general,
provided they are not of a black colour. Tartaric acid, sulphate of
quinine, and nitrate of uranium increase this sensibility. The paper
which has been impressed preserves its undeveloped image for a prolonged
period if kept in darkness; and it has been found that one piece of
paper can impart the image to another by simple contact in the dark.
What is still more remarkable, the invisible impressions on a piece of
paper may be transferred to another not in contact by merely placing it
opposite the first, and separated by an interval of a quarter of an
inch. No satisfactory explanation of these phenomena has been advanced,
but many conjectures have been made. One of these supposes that some
unknown intermediate products are formed, which are, in the case of the
latent image on paper, very oxidizable; but in the case of silver salts,
&c., very reducible, so that the addition of a silver salt in the first
case, and of organic matter in the second, only completes the phenomena
by ordinary chemical action. Niepce de Saint-Victor, however, found that
a surface of freshly broken porcelain alone will receive a latent
impression from light, and will reduce in those places sensitive salts
of silver. He believes that the light in these latent images is simply
stored up, and that its energy remains fixed to the surfaces until the
occasion of its producing a chemical action.
When a pure solar spectrum is made to fall upon paper rendered sensitive
by silver salts, the effect is observed to be greatest near the
Fraunhofer line H (No. 1, Plate XVII.), and it is prolonged with
decreasing intensity beyond the violet end of the spectrum, while
towards the other end it terminates about the line F. When other
sensitive substances are used, the range of photographic power in the
spectrum is modified. It has been found that when a daguerrotype plate
which has been impressed by the light in the camera is afterwards
exposed to the red or yellow rays of the spectrum, it loses its property
of condensing the mercurial vapours. This destruction of photographic
impression by red or yellow light has a practical application of great
importance, for it permits the processes of preparing paper and plates
to be carried on in a laboratory lighted by windows having yellow or
red, instead of the ordinary colourless, glass. Thus we see that it is
by no means the whole of the solar rays which are concerned in producing
photographic images; nay, there are some which even tend to destroy the
impressions produced by others. The fact that it is not the light, but
only certain rays in the sunbeam, may be proved very conclusively by an
experiment with a glass bulb filled with a mixture of equal volumes of
hydrogen and chlorine gases. When such a bulb is exposed to the light of
the sun or of burning magnesium, which is made to reach it by passing
through a piece of _red_ glass, no explosion takes place; but if the
bulb be covered only with a piece of _blue_ or _violet_ glass, the
explosion is produced just as quickly as if it were exposed to the
unaltered rays.
The visible spectrum obtained in the experiment described on page 318 is
far from constituting the only radiations which reach us from the sun.
For invisible beams of heat, less refrangible than the red rays, are
found beyond the red end of the spectrum; and another invisible spectrum
stretches far beyond the violet end, formed of rays recognized only by
their chemical activity. It is these which effect photographic actions,
and though they are in part more highly refrangible than any of the rays
producing the visible spectrum, a large portion are refracted within its
limits, so that the maximum of photographic action in a spectrum is
usually near the violet end. When we wish to examine the spectrum of the
heat rays, it is necessary to replace the glass prism by one made of
rock salt, for glass absorbs these heat rays. It also intercepts a great
part of the most refrangible rays; for when a prism of _quartz_ is
substituted for the glass one, the spectrum becomes greatly extended at
the violet end. The dark Fraunhofer lines which cross the visible
spectrum are represented also in great numbers in the invisible
spectrum: in photographs of the _ultra-violet_ rays more than 700 dark
lines have been counted. It has been proposed to employ quartz lenses in
the photographic camera; but there is reason to believe that the
increased transparency of such lenses for the chemical rays would be
counterbalanced by certain disadvantages attending the use of quartz.
The beauty of the images which are formed in the camera obscura long ago
gave rise to the desire of fixing them permanently. We know how
perfectly photography has already satisfied that desire, so far as the
_forms_ are concerned. The very perfection of the results obtained in
this direction increases our regret at our inability to fix also the
colours, and secure the picture, not in grey or brown tones of reduced
silver, but with all the glowing hues of nature. An observation made by
Herschel, Davy, and others, seemed at one time to hold out hopes of a
possible realization of chromatic photographs. It was noticed that the
images developed upon chloride of silver, of the different parts of the
solar spectrum, partook somewhat of the colours of the rays which
produced them. Edmond Becquerel made a plate of polished silver, placed
in dilute hydrochloric acid, form the positive pole of a battery. The
plate thus became coated with an extremely thin layer of chloride of
silver, which, as its thickness augmented, exhibited the series of
colours due to the action of light on thin films. The operation was
stopped when the plate had become of a violet colour for the second
time; it was then washed, dried, polished with the finest tripoli, and
heated to 212° F., the whole of these operations having been carried on
in the dark. When this plate was exposed for about two hours to the
solar spectrum, fixed by proper appliances which counteracted the
apparent motion of the sun, the luminous rays were found to have
impressed the plate with their respective colours. The yellow was
somewhat pale, but the red, green, and violet were exhibited in their
true tints. A theoretical explanation has been advanced, which supposes
that yellow light, for example, renders the surface of the plate on
which it falls peculiarly capable of receiving and transmitting
vibrations corresponding to those of yellow light. Just as a stretched
cord responds to its own musical note, the modified plate gives back,
out of all the vibrations which fall upon it in ordinary light, only
those of which it has itself acquired the periodicity. But since the
plate has not lost its sensitiveness to take on other rates of
vibrations, it receives other impressions, which first weaken and then
overcome the former, and, therefore, the colour necessarily vanishes.
This kind of difficulty seems to be a necessary concomitant of every
attempt in this direction; and all the hopes founded on results yet
obtained have been disappointed by the rapid fading of the images.
The comparative cheapness and convenience of Talbot’s process, and
especially the facilities which it afforded for the multiplication of
proofs, gave an immense impulse to photographic art. But the irregular
and fibrous structure of paper prevented the attainment of the beautiful
sharpness of outline and clear definition of detail which the plates of
Daguerre presented. Sir John Herschel suggested the use of glass plates
coated with sensitive photographic films, and Niepce de Saint-Victor
succeeded in fixing upon glass layers of albumen (white of egg)
containing the silver salts, a method which is still used to some
extent. The art received, however, its greatest stimulus from the
improvements which ensued on the application of _collodion_ to this
purpose. Collodion (κολλα, glue; in allusion to its adhesiveness) is the
name which has been given to a solution in ether of gun-cotton, or of a
substance nearly allied to it. Its employment was suggested by Le Grey
of Paris, but the late Mr. Archer was the first to carry the idea into
practice, and the process which he described in “The Chemist,” in 1851,
is virtually that which is now almost universally adopted. This process
has now been tested, for nearly a quarter of a century, by the united
experience of photographers all over the world, and it is agreed that it
is surpassed by no other, for it secures every quality which a
photograph can possess.[13] The minor details of the method can be, and
are, infinitely varied; scarcely two experienced photographers will be
found working the process in identically the same manner throughout.
Before giving an outline of the collodion process, it may be well to say
something respecting the chief instrument of photography—the camera.
Footnote 13:
(1875) But see below, page 541.
[Illustration:
FIG. 308.
]
The ordinary photographic camera is almost too well known to require
description. In its simplest form, Fig. 308, it is merely a rectangular
box, in front of which is placed the lens, which slides in a tube, that
its position may be adjusted so as to bring the rays to a focus on the
surface of a piece of ground glass at the opposite end. This glass is
fitted into a light frame, which slides in grooves, so that it can be
raised vertically out of its position, and replaced by another frame, B,
which contains a recess for the reception for the sensitive plate, and a
sliding screen which protects it from light until the right moment. When
this frame is placed in the camera, the sensitive surface occupies the
same position as that of the ground glass, and the sliding screen is
drawn up the moment before the operator removes from the front of the
lens a cap which he places there after adjusting the focus. The sliding
screen is usually made with a narrow strip at the lower part, joined to
the rest by a hinge, so that when it has been drawn up it may be
retained in its position, and placed out of the way, by being folded
down horizontally. There is commonly provision for two plates in one
frame, the slides, &c., being doubled, and the plates placed back to
back, as shown at B, Fig. 308. The camera is usually made in two parts,
as shown in the figure, that at the back sliding within the other, so
that a wider range for adjustment is obtained, and the same camera may
even be used with lenses of different focal lengths. Many improvements
have been made in the camera, by which it has been rendered more
portable, and capable of more adjustments to suit varying circumstances.
Fig. 309 represents a “bellows” or folding camera, which appears to
supply every requirement for the studio. It is copied from Messrs.
Negretti and Zambra’s catalogue, as are also the other figures of
photographic apparatus here given. Fig. 307 represents a camera for
taking stereoscopic views, fitted with two lenses, so that the two views
are taken simultaneously on one plate.
[Illustration:
FIG. 309.
]
No piece of apparatus used by the photographer is of so much importance
as the lens; for good pictures cannot be obtained without well-defined,
sharp images on the sensitive plate, and these images must have
sufficient intensity to produce the required amount of chemical action
in a short space of time. The formation of an image by means of a lens
which is thickest at the centre is tolerably familiar to everybody; for
most persons must have noticed that the lens of a pair of spectacles, or
of an eye-glass, will produce an inverted image of the window-frame on a
sheet of white paper, held a certain distance behind the lens. But the
diagrams by which the paths of the rays are usually represented seem to
convey a false impression to an ordinary reader, who usually goes away
with the idea that somehow three rays are sent off by the object, and
that one goes through the middle of the lens, and the other two meet it
and produce an image. Let us suppose that, by means of a circular
eye-glass, the image of a window is projected on a piece of white paper:
a straight line passing through the centre of the glass perpendicular to
its plane will meet the window and image each at a certain point. The
point in which it meets the image is the _focus_ of _innumerable_ rays,
which issue from the point in the window; that is, of the whole light
sent out in every direction by the point a certain portion falls upon
the lens, and by the refraction it undergoes in passing through it, the
rays are again brought together at the point in the image. Thus the
original point in the object is the apex of a solid cone of rays (if we
may say so), of which the lens is the base, and the point in the image
is the apex of another cone, having also the lens as its base. These
cones would be termed _right_ cones, because their bases are
perpendicular to their _axes_, or central lines. But they represent the
rays from only _one point_ of the object. Let us now consider how the
image of another point is formed, say one in the highest part of the
object which forms an image on the screen. Those rays which are sent out
by this point, and fall upon the lens, form now an _oblique_ cone, of
which the lens is the base, and the central ray will pass through the
middle of the lens and continue its journey on the other side with
little or no change of direction, forming also the axis of another
oblique cone, constituted of the refracted rays, all of which will meet
together at the lowest part of the image. Similar cones of incident and
refracted rays, all having the lens as base, and all of them cones more
or less oblique, will be formed by the light from each point of the
object. Thus, the rays which issue from each point are brought together
again in a series of points which have the same position with regard to
each other, and collectively form an inverted image.
On carefully looking at the image, say of a window-frame, formed by a
simple lens, the reader will observe two defects. The first is that the
image cannot be made equally clear and well defined at the centre and at
the edges: the adjustment which gives clear definition of one part
leaves the other with blurred outlines. The second defect, which is best
seen with large lenses, consists in coloured fringes surrounding the
outlines of the objects. This depends upon the unequal refrangibility of
the various rays, but it is obviated in _achromatic_ lenses, which are
formed of two or more different kinds of glass, so adapted that the
refracting power of the compound lens is retained, and the most powerful
rays of the spectrum are brought to a common focus. Such are the lenses
always used in the photographic camera, and the skill of the optician is
taxed to so combine them as to obtain, not only the union of the
principal rays in one focus, but the greatest possible flatness of field
in the image, the largest amount of light, the widest angle without
distortion of the picture, and other qualities.
[Illustration:
FIG. 310.
]
Photographers have even been so fastidious in the matter of lenses as to
require all the perfection of finish which is given to the
object-glasses of astronomical telescopes. Mr. Dallmeyer has made
photographic lenses which cost upwards of £250; but it is doubtful
whether the pictures formed by these would show any marked superiority
over those produced by lenses costing only one-fifth of that amount.
Fig. 310 shows the construction of the combination usually employed for
taking photographic portraits. A is a section showing the forms and
positions of the different lenses; B is an external view of the brass
mounting of the lens. It is provided with a flange, C, which is attached
by screws to the woodwork of the camera; and within the short tube, of
which this is a part, slides the tube carrying the lenses, being
furnished with a rack and pinion moved by the milled head, E. D is a cap
for covering up the front of the sliding tube. A slit in the tube admits
of plates of metal, perforated with circular openings, being inserted.
The openings are of various sizes; and these “stops” or diaphragms
enable the operator to regulate the amount of light; and to cut off when
required the rays passing through the marginal parts of the lens.