was, and still is, much practised, namely, the _collodion process_. The
collodion solution is prepared by dissolving one part of pyroxylin
(gun-cotton) in ninety parts of ether and sixty of alcohol. The
pyroxylin for this purpose may be obtained by steeping cotton-wool for a
few minutes in a mixture of nitre and sulphuric acid, with certain
precautions which need not here be mentioned. To the solution of
collodion is added a certain quantity of iodide of potassium, or of
iodide of ammonium; and sometimes other substances also are mixed with
the solution with a view of increasing the sensitiveness of the plate
when ready for exposure. Some of the collodion solution is poured on a
well-cleaned plate of glass, which is placed horizontally; it spreads
over the plate, and the excess having been poured back into the bottle,
the evaporation of the liquids leaves the glass covered with a thin
uniform transparent film, which firmly adheres. The next operation is to
render the plate sensitive by means of the “silver bath.” This is a
neutral solution of nitrate of silver, one part to fifteen of pure
water, which is placed in a trough of glass or porcelain, Fig. 311. By
the aid of a proper support the plate is introduced quickly and steadily
into the solution, immediately after the collodion film has been formed
on its surface. In two or three minutes the layer of collodion becomes
impregnated with iodide of silver, and when taken out of the bath, the
plate exhibits a creamy-looking surface. The operation of sensitizing
the plate by the silver bath must be performed in a room to which no
light has access, except that which has passed through _red_ or _yellow
glass_, or a semi-transparent yellow screen.
The plate is now ready for immediate exposure in the camera. It is
placed in the dark slide, in which it is conveyed to the camera; and
there the image of the object is allowed to fall upon it for a time,
which varies, according to the intensity of the light and the nature of
the object, from 3 seconds to 45 seconds. The slide is withdrawn from
the camera, and taken again to the “dark” room, _i.e._, where only
_yellow_ or _red_ light can reach it. If the plate be now examined, it
will be found to present no trace of an image. A latent one, however,
exists; and it is developed by pouring over the plate a solution of
pyrogallic acid—one part to 480 of water, with commonly a little alcohol
and acetic acid added. When it is desired to intensify the image still
more, a few drops of the nitrate of silver solution is added to the
_developing solution_ immediately before pouring it on the plate. When
the picture has become sufficiently distinct, it is washed with pure
water, and then immersed in a strong solution of hyposulphite of soda.
The last operation is termed by photographers “fixing” the picture, and
the substance employed in it is invaluable to the art. It acts as a
ready solvent of all the salts of silver which remain on the plate; and
the discovery of this property of the hyposulphites by Sir J. Herschel,
in 1839, marked an era in photography. The picture is then thoroughly
washed in cold water, in order that the hyposulphite of soda may be
entirely dissolved out. It is then dried, warmed before a fire, and
finally the film is covered with a coat of transparent varnish, by which
it is protected from mechanical injury. The image here is
_negative_—that is, the strongest lights of the object appear as the
darkest tints in the picture, and _vice versâ_. From it any number of
_positive_ pictures may be obtained by means of the sensitive paper
prepared with chloride of silver as in Fox Talbot’s plan.
As it is a tedious, and perhaps, in some cases, an impossible operation
to completely remove all traces of silver salts and hyposulphites from
photographs, they have frequently been found to fade; but this is rarely
the case with well-prepared specimens. Processes have, however, been
devised by which absolute permanence is secured for the photograph. One
of the best of these is known as the Carbon Printing Process, and, as
improved by Mr. Swan, it is thus practised:
A solution of gelatine is coloured by the addition of Indian ink, or any
other pigment which will give the desired tone. This solution is spread
over sheets of paper which are then dried. In this condition the paper
may be preserved for any length of time without any special precautions.
When it is required for use, it is floated, with the gelatine-covered
side downwards, in a solution of bichromate of potash, and then dried;
but these operations must be carried on in the dark. The paper is
exposed under a negative photograph, with which its prepared side is in
contact. The effect of the light is to render insoluble the gelatine on
all those parts on which it has fallen, and this action extends to a
depth in the layer proportionate to the intensity of the illumination.
The object is, therefore, to wash away all the _soluble_ gelatine and
the colour with which it is mixed; but this soluble gelatine is mainly
on the side of the film which is in contact with the paper. The gelatine
surface is therefore made to adhere to another piece of paper by means
of some substance insoluble in water; and when this has been done, the
whole is immersed in warm water. Then the soluble gelatine is soon
dissolved; the first paper floats off, and the insoluble gelatine,
holding the Indian ink or other colouring matter in its substance,
remains attached by the cement. As the thickness of the layer rendered
insoluble is in proportion to the intensity of the light passing through
each part of the negative, the picture will be presented in all the
proper gradations of light and shade.
[Illustration:
FIG. 311.
]
The “wet collodion” process, that has been described on the preceding
page, maintained an almost undisputed hold for more than twenty years in
the practice of photography in all branches, and it was not until after
the publication of the first edition of the present work that a new era
in the art was commenced by the introduction of what is known as the
_dry plate gelatino-bromide process_, to which the present enormous
popularity of photography as a recreative art is due. The difficulties
of manipulation, the necessity for extensive experience, and for special
and cumbersome appliances were obstacles it at once removed. And not
only so, but the whole scope of the art was extended; for work that was
before supposed impracticable, even to the most expert professional
photographer, became the amusement of the amateur. Here, we may remark
in passing, that photography is greatly indebted for this, and many
other improvements, to the enthusiasm of the amateur, which has
accelerated the development of the art to a remarkable extent. The
collodion process itself admitted of being modified as a dry plate
method, by coating the film with a preservative solution of tannin, gum,
albumen, or other substance, and then drying the plates, of course in a
dark place. This plan made it possible to practise out-door photography
with ease, and such plates were, at one time, much used for landscape
photography, but they have now been almost superseded by the gelatine
plates. It was Mr. Bennet, who, in 1874, first introduced the use of
sensitive emulsions of gelatine, and the advantages offered by their
use, caused them to be soon adopted by landscape and amateur
photographers. In 1878, Mr. Bennet showed, that these plates could be
made wonderfully rapid in their action, so that portraits, etc., could
be taken by them in an unprecedentedly short time. The preparation of
the dry gelatine plates was then commenced on a large scale, and these
were found so convenient, and reliable in use, that they were adopted by
the professional photographers, who had hitherto adhered to the wet
collodion and silver bath, from long habit and established associations.
The collodion processes are, however, still much used, and are preferred
by many to the gelatine plates; indeed, it is admitted, that only by the
former can certain desirable qualities of negatives be obtained, which
are of great importance in some applications of the art.
There are, it need hardly be said, many modifications of the processes
recommended for preparing gelatino-bromide dry plates, and each
manufacturer of the various kinds offered for sale has, no doubt, his
own special plan and formula. In all, a very fine and carefully selected
quality of gelatine is the medium in which the sensitive salts are
embedded. An “emulsion” is prepared by adding to warm gelatine solution
exactly determined quantities of solutions of certain compounds, of
which a bromide (usually bromide of potassium) and silver nitrate are
the essential ones, together with a small proportion of iodide of
potassium. Minute quantities of iodine, hydrochloric acid, etc., are
also often prescribed as additions. The mixture has to be heated, at the
boiling temperature, for three quarters of an hour, then cooled, and
mixed with more gelatine solution, or, instead of using acid and iodine
and boiling, a little ammonia is added. When cold and set, the gelatine
is washed with cold water, while squeezed through canvas, or after it
has been cut into thin strips. It is then drained, dissolved at a gentle
heat, and filtered warm. The clean glass plates are coated over with it,
at the temperature of 120° F., and are set aside in a perfectly
horizontal position until the gelatine has set, when they are placed for
twenty-four hours in a drying cupboard, maintained at 80° F. It will be
understood that these operations are conducted in a room where no light
enters, except through a frame of ruby-coloured glass, and the plates,
when dry, are carefully packed and stored in light-tight boxes. They are
marvellously sensitive, and receive the photographic impression in about
one-sixtieth (1/60th) of the time required for wet collodion plates.
Half a second exposure in the camera may be sufficient to impress the
image of a well lighted landscape, even when a very small stop is used,
and it is not unusual to employ for extra sensitive plates, a so-called
“instantaneous shutter,” when the exposure may be no more than 1/80th to
1/100th of a second, and yet obtain a perfectly strong image. Dry plates
are manufactured in vast numbers in many large establishments, and the
operations are carried on to a great extent by the aid of machinery, by
which the plates are uniformly coated and automatically carried into
drying chambers, etc.
If photography were popular before the introduction of the dry
gelatino-bromide plates, it has since become a hundred-fold more so.
Indeed, the camera is now seen everywhere, and few are the family
circles in which at least one amateur practitioner of the art is not to
be found; indeed, the technical terms of the art have become “Familiar
in their mouths as household words.” The daguerrotype, notwithstanding
its cost, had no sooner become a practicable process for taking
likenesses, than it began to supersede miniature painting, and how
rapidly it rose into general favour may be inferred from the fact that,
in 1850, ten years after its introduction, it was estimated that in the
United States of America, at least ten thousand persons had made it
their profession, and, probably half as many more were occupied in
making and selling chemicals, plates, cameras, lenses, mounting cases,
and other apparatus connected with its practice. Such being the demand
for photographic portraits, at the period when the sitter had, as we
have already seen, to remain motionless for two whole minutes in
sunlight, we can hardly be surprised at the increased popularity the art
has acquired in the last decade, when a picture can be produced with
one-hundredth the length of sitting, and at about the same reduction of
cost. It may here be mentioned, that Daguerre’s process is still
occasionally used for special purposes; it was, for instance, the method
selected for obtaining the photographic records in the expedition sent
out by the French Government, in 1874, to observe the transit of Venus.
[Illustration:
FIG. 311_a_.—_The Roll-Slide._
]
The dry plate processes have given an immense impulse to landscape
photography, and travellers have been able to bring back authentic
representations of the scenery and inhabitants from every part of the
globe. This advantage arises from the fact that having the camera, and
its appurtenances, the tourist or traveller is not obliged to carry
anything about with him except his plates, and when these have once been
exposed in the camera, and stowed away in light-tight boxes, the latent
images may be developed months, or even years, afterwards. But glass
plates are heavy, and are liable to accidental breakage. Inventive
ingenuity has been actively at work for the past few years, to find a
means of obviating these remaining inconveniences. The first method
adopted was to employ paper instead of glass, as a support for the
sensitive gelatine film. The paper, having been cut to the proper size,
is placed on a _film-carrier_, which is usually a thin plate of ebonite,
by which the paper is kept flat. These carriers take the place of the
glass plates in the ordinary dark slide, and after exposure in the usual
way, the papers are removed in the dark room and made up into
light-tight packages, where, of course, a large number will occupy but a
small space, and the weight of them be wholly negligible. Many persons
make use of this arrangement, which has the advantages of simplicity and
of requiring no special apparatus. But an improvement was soon brought
out, which consists in substituting for the carriers and pieces of
sensitive paper a continuous roll of the material. For this purpose a
special piece of apparatus, called the roll-holder, is made to take the
place of the dark slide at the back of the camera. The arrangement will
be readily understood from Fig. 311_a_. The figure shows the apparatus
in section, but only the disposition of the principal parts, most of the
mechanical details being omitted. R R´ are two metallic or wooden
rollers, which admit of being readily put in their places and taken out.
Upon one of these, R, the full length of the material is previously
wound, and the free end is passed over another roller, _r´_, and across
the opening at E O, where the exposure is made. There is in front of
this a dark slide (not here shown) to be drawn up when everything is
ready for uncovering the lens. Immediately behind the paper is a flat
plate of ebonite, E, or a smooth black board, the object of which is to
keep the material quite flat as it passes over the opening to the
roller, _r´_, which guides it to the roll, R´, on which it is wound as
required. S S´ are two small rollers always pressed by springs against
the rolls to prevent the turns working loose. There is a registering
apparatus outside in connection with one of the rollers, _r_, or _r´_,
to show when the proper length of material has been wound across the
opening for a new exposure; and at the same time a mark is automatically
made on the paper to indicate where the negatives are to be separated
for development by cutting the paper. Some forms of the apparatus also
call the operator’s attention to the sufficient winding of the roll by
an audible signal, a stroke on a little bell tells that everything is
ready for a new exposure. In some cases the number of exposures already
made is registered by figures that appear on the outside. The paper in
these processes is used only as a temporary support; for after the
negative has been developed in the ordinary way, the sensitive gelatine
film is removed from it and made to adhere firmly on a plate of clear
glass, from which prints are taken as usual. The operations required for
the transferring require considerable dexterity of manipulation, and to
both the paper and the glass special preparations have to be applied,
before and after the transference of the film. This plan, therefore, of
“stripping films” involves so great a number of delicate and somewhat
troublesome operations that very many photographers have preferred to
encounter the labour and risks of carrying about with them the more
easily manageable glass plates. But what if some grainless, transparent
substance could replace the paper in these rolls so that the negatives
might be ready for printing from when merely developed and fixed? Many
trials have been made to find this desideratum. A material sufficiently
translucent, even, and of tenacity enough to bear the stretching strain
between the rollers has, it is believed, been discovered in a very
singular substance previously used for other purposes. The reader is no
doubt familiar with it as the substitute for ivory in combs, knife
handles, and other small articles. It is called _celluloid_, and is a
composition the principal ingredients of which would never be guessed
from its appearance—namely gun-cotton and camphor! This material is
prepared in a plastic condition that enables it to be shaped into any
required form. It can be drawn into threads or rolled out into very thin
films. Thin plates of it have been used in photography as a substitute
for glass, for the sake of lightness, before its employment as a
transparent film in the roll-holders. We have now at length the
equipment of the travelling photographer reduced to the utmost
conceivable limits of lightness and compactness. Thus the complete
apparatus required for taking hundreds of pictures of a good size need
not be more than a few pounds in weight, and can easily be carried in
the hand. But even quite small negatives can now be very readily printed
in a few seconds on paper, with an enlargement of many times the
original dimensions. The resources of the photographic art appear indeed
to be endless; but a mere statement of even the more interesting of
these would lead us beyond our limits, and descriptions of the details
of manipulation are out of our province altogether. But a few of the
more recent applications and developments of the art scarcely or not at
all alluded to in the foregoing pages should receive some attention.
The extraordinary sensitiveness of the gelatine-bromide film which makes
it possible to impress on it a photographic image in the merest fraction
of a second of time, enables us to take pictures of objects in rapid
motion. Express trains at their highest speed have been successfully
photographed, and so has almost every moving object in nature. The
photographs that have been taken of men, of birds, horses, and other
animals in every phase of their most rapid actions, have solved many
disputed and perplexing problems as to the nature of their movements,
and sometimes the solutions have been of a very unexpected kind. Taking
a photographic “shot” at a bird has become almost more than a figure of
speech; for there are contrivances by which a bird on the wing may be
aimed at with the lens, and hit off on the sensitive plate with a
certainty surpassing that of the fowling-piece. There are also
photographic repeaters by which six or more successive photographs of
the bird, etc., can be taken in a single second. Mr. Muybridge has
published a number of such photographs of the horse, and by projection
of the different images on a screen from a magic lantern, in rapid
succession, he has been able to reproduce the visual appearance of
horses trotting, leaping, galloping, etc., on the principle of the
zoetrope (page 399). Photography has afforded wonderfully delicate
observations in many departments of science, by recording phenomena too
rapid for the eye to seize, or too recondite for direct perception. A
few examples may be mentioned. First, the advantage of photographing the
lines of spectra, such as those described in our article on the
spectroscope, will at once suggest themselves, and accordingly this
method of recording spectra has been largely used, and in the hands of
Mr. Lockyer, Dr. Draper, and others has been successfully applied to the
study of the solar and stellar spectra. But more than this, it is the
sensitive photographic plate that has enabled us to explore the region
of the solar spectrum lying far beyond its visible limits in the red and
in the violet rays. The ultra-violet portion of the spectrum is shown
photographically to be occupied by multitudes of the thin insensitive
spaces—breaks in the continuity of the active rays—which are impressed
on the photographic print as black lines, similar in every respect to
the lines mapped out in the visible spectrum by Fraunhofer. It is known
by these that the ultra-violet spectrum, produced by glass prisms,
extends to a distance beyond the last visible rays of nearly double the
space occupied by the colour spectrum. The principal lines, or rather
the greater groups of lines in the invisible spectrum, are distinguished
by the capital letters of the alphabet, in continuation of Fraunhofer’s
method, beginning from H and nearly exhausting the letters of the
alphabet to designate them. These are photographed _in the dark_; for
all the solar beams that are allowed to enter the stereoscope are first
passed through blue glass of such a depth that every kind of emanation
capable of affecting the human eye is intercepted.
Another extremely interesting example of the application of the art to
scientific research is celestial photography. An image of the sun may be
impressed on a sensitive plate in an ordinary camera, in an amazingly
short space of time, but such image is much too small to show any of the
markings on the disc of our luminary, even when the image is magnified,
for its diameter is only about ⅒th of an inch for each 12 inches of the
focal length of the lens. In order to obtain an image of 4 inches
diameter, a lens of 40 feet focal length must therefore be used. The
first attempts in solar photography appear to have been made in France,
in 1845, and the solar prominences were daguerrotyped in 1851; but it
was not until 1860, that Mr. De La Rue succeeded in obtaining some
beautiful negatives of the phenomena presented in an eclipse of the sun,
and was thus enabled to determine a great astronomical problem, by
showing that the red flames, or prominences, really belonged to the sun
itself. At the present time, photographs of parts of the sun’s disc are
regularly taken at Kew, and other observatories, without the very long
and heavy telescopes, which introduced many mechanical difficulties into
the operation; for, by means of Foucault’s siderostat, the great lens
and the photographic apparatus can be used in one fixed position. The
siderostat is an instrument on which a flat mirror, made of glass worked
to a perfect plane and silvered externally, is caused by clockwork to
follow the motion of the sun, so that the reflected beams can be
projected in any required direction unchangeably, and, therefore the
image of the sun (or other heavenly bodies) viewed in the mirror, is
absolutely stationary. The lens, carried in a short tube, has its axis
directed to this image, just as it would be pointed at the luminary
itself. In solar photography, the exposure is made through a very narrow
slit in an opaque screen, which is caused to move rapidly in front of
the image. Very fair photographic images of the sun, of several inches
diameter, can, however, be obtained with an ordinary telescope of five
feet or so focal length, by substituting a small photographic lens and
camera in the eye-piece, and by enlarging the image in printing.
As early as 1840, Dr. Draper succeeded in daguerrotyping the moon, but
it was not until 1851, that lunar photographs, obtained by Professor
Bond, another American astronomer, were first exhibited in England. Many
other distinguished experimenters have since successfully turned their
attention to this subject, such as Dancer, of Manchester, Secchi,
Crookes, Huggins, Phillips, and De La Rue. The latter, and also Mr. Fry,
by photographing the moon, at different periods of her libration, have
obtained very beautiful and interesting _stereoscopic_ prints of our
satellite, in which she presents to the eye the roundness and solidity
of a cannon ball. Mr. Rutherford, in America, had an object glass of 11¼
inches diameter, made expressly with correction for the chemical rays,
and with this instrument he has produced some of the finest photographs
of the moon that have yet been taken. Reflecting telescopes, which have
the advantage of uniting all the rays in one focus, have been used with
excellent results, and it is said that some taken with the great
reflector at Melbourne, where also the atmospheric conditions are very
favourable, are almost perfect.