PHOTOGRAPHY.
EXPERIMENTS OF WEDGEWOOD AND DAVY--NIÉPCE’S HELIOGRAPHY--DAGUERRE
AND THE DAGUERREOTYPE--FOX TALBOT MAKES FIRST PROOFS FROM
NEGATIVES--SIR JOHN HERSCHEL INTRODUCES GLASS PLATES--THE COLLODION
PROCESS--SILVER AND CARBON PRINTS--AMBROTYPES--EMULSIONS--DRY
PLATES--THE KODAK CAMERA--THE PLATINOTYPE--PHOTOGRAPHY IN COLORS--
PANORAMA CAMERAS--PHOTO-ENGRAVING AND PHOTO-LITHOGRAPHY--HALF TONE
ENGRAVING.
“Art’s proudest triumph is to imitate nature.”
When nature paints she does so with the brush of beauty, dipped in the
pigment of truth. The tender affection of a ray of light touches the
heart of a rose, brings a blush to its cheek, and life, becoming the
bride of chemical affinity, blooms into surpassing beauty and
loveliness. Photography is closely allied to nature’s painting, for just
as light brings into existence nature’s living beauties, so does light
fix, preserve, and perpetuate these beauties by the same subtile and
mysterious agency of a quickened chemical affinity. Photography is both
an art and a science, and as such is both beautiful and true. It is an
art intimately associated with the tenderest affections of the human
heart in keeping alive its precious memories. By it the youthful
sweetheart of long ago, the loving face of the departed mother, and the
cherished form of the dead child are brought back to us in familiar
presence, while our great men have become the every-day friends and
ideals of the common people. What an enrichment and satisfaction it
would have added to our lives if the art had been coeval with history,
and all the world’s exalted scenes and faces had come to us through the
camera with the knowledge of absolute truth and fidelity. But not only
in portraiture is photography a great art, for it catches the stately
pose of the mountain, the grandeur of the sea, the beauty of the forest,
or the majesty of Niagara Falls, and brings them all home to us, even to
the vision of the bed-ridden invalid. The camera alike records the
secrets of the starry heavens and the bacteria of the microscopic world.
Hanging on the tail of a kite it photographs the face of mother earth,
and, acting quicker than the lightning, it catches and defines the path
of that erratic flash. It plays the part of a private detective, and its
testimony in court is never doubted. The architect, engineer, and
illustrator find it in constant requisition. By the aid of the Roentgen
Rays, it locates a bullet in a wounded soldier, and takes a picture of
one’s spinal column. In fact, it sees and records things both visible
and invisible, acts with the rapidity of thought, and is never mistaken.
The art of photography, named from the two Greek words φωτος γραφη (the
writing of light), is a comparatively new one, and belongs entirely to
the Nineteenth Century. It was known to the ancient alchemists that
“horn silver” (fused chloride of silver) would blacken on exposure to
light, but there was neither any clear understanding of the nature of
this action, nor any application made of it prior to the year 1800. We
now know that the art of photography is dependent upon the actinic
effect of certain of the rays of the spectrum upon certain chemical
salts, notably those of silver and chromic acid, in connection with
organic matter. The rays which have this effect are the blue and violet
rays at one end of the spectrum, and even invisible rays beyond the
violet, the red and yellow rays having little or no such actinic effect.
That which made photography possible for the Nineteenth Century was the
philosophical observation of Scheele, in 1777, upon the decomposing
influence of light on the salts of silver, and the superior activity of
the violet rays of the spectrum over the others in producing this
effect. In 1801 Ritter proved the existence of such invisible rays
beyond the violet end of the visible spectrum by the power they
possessed of blackening chloride of silver.
_Earliest Application of Principles._--The first attempt to render the
blackening of silver salts by light available for artistic purposes, was
made by Wedgewood and Davy in 1802. A sheet of white paper was saturated
with a solution of nitrate of silver, and the shadow of the figure
intended to be copied was projected upon it. Where the shadow fell the
paper remained white, while the surrounding exposed parts darkened under
the sun’s rays. There was, however, no means of fixing such a picture,
and in time the white parts would also turn black.
_Introduction of Camera._--The camera obscura, a very old invention
designed for the use of artists in copying from nature, was at a very
early period brought into this art, but it was found that the chemicals
employed by Wedgewood and Davy were not sufficiently sensitive to be
affected by its subdued light. In 1814, however, Joseph Nicéphore
Niépce, of Chalôns, invented a process that utilized the camera, and
which was called “Heliography,” or sun drawing. In 1827 he discarded
the use of silver salts, and employed a resin known as “Bitumen of
Judea” (asphaltum). A plate was coated with a solution of this resin and
exposed. The light acting upon the plate rendered the resin insoluble
where exposed, and left it soluble under the shadows. Hence, when
treated with an oleaginous solvent the shadows dissolved out, and the
lights, represented by the undissolved resin, formed a picture, which
was in reality a permanent negative. The process, however, was slow,
requiring some hours.
_The Daguerreotype._--In 1829 Niépce and Daguerre became partners, and
in 1839, after the death of the elder Niépce, the process named after
Daguerre was perfected (British patent No. 8,194, of 1839). He abandoned
the resin as a sensitive material, and went back to the salts of silver.
He employed a polished silver surfaced plate, and exposed it to the
action of the vapors of iodine, so as to form a layer of iodide of
silver upon the surface, which rendered it very sensitive. 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 vapor of mercury. This
process reduced the time required from hours to minutes, and as it
involved the production of a latent image, which was subsequently
developed by a chemical agent, it represented practically the beginning
of the photographic art as practiced to-day. Daguerre sought also to
permanently fix his pictures, but this was accomplished only imperfectly
until 1839, when Sir John Herschel made known the properties of the
hyposulphites for dissolving the salts of silver. In 1844 Hunt
introduced the protosulphate of iron as a developer.
_Production of Positive Proofs from Negatives._--This was first done by
Mr. Fox Talbot, of England, between 1834 and 1839. In his first
communication to the Royal Society, in January, 1839, it was directed
that the paper should be dipped first in a solution of chloride of
sodium, and then in nitrate of silver, which, by reaction, produced, on
the face of the paper, chloride of silver, which was more sensitive to
the light than nitrate of silver. The object to be reproduced was laid
in contact with the prepared paper, and exposed to the light until a
copy was produced which was a negative, having the lights and shadows
reversed. A second sheet was then prepared, and the first or negative
impression was laid upon it, and used as a stencil to produce a second
print which, by a reversal of the lights and shadows, formed an exact
reproduction of the original. In 1841, British patent No. 8,842 was
obtained by Mr. Talbot, for what he called the “Calotype,” and which was
afterward known as the “Talbotype.” A sheet of paper was first coated
with iodide of silver, by soaking it alternately in iodide of potassium
and nitrate of silver, and was then washed with a solution of gallic
acid containing nitrate of silver, by which the sensitiveness to light
was increased. An exposure of some seconds or minutes, according to the
brightness of the light, produced an impression upon the plate, which,
when treated with a fresh portion of gallic acid and nitrate of silver,
developed into the image. After being fixed it formed a negative from
which any number of prints might be obtained. The Talbot process
represented a great advance in this art. Glass plates to retain the
sensitive film were introduced by Sir John Herschel in 1839, and were a
great improvement over the paper negatives, which latter, from lack of
transparency and uniformity in texture, had prevented fine definition
and sharpness of outline. Blue printing was also invented by Sir John
Herschel in 1842, and he was the first to apply the term “negative” in
photography. In 1848 M. Niépce de St. Victor, a nephew of Daguerre’s
former partner, applied to the glass a film of albumen to receive the
sensitive silver coating.
_Collodion Process._--The most important step in the preparation of the
negative was the application of collodion. This is a solution of
pyroxilin in ether and alcohol, which rapidly evaporates and leaves a
thin film adhering to the glass. M. Le Gray, of Paris, was the first to
suggest collodion for this purpose, but Mr. Scott Archer, of London, in
1851, was the first to carry it out practically. A clean plate of glass
is coated with collodion sensitized with iodides of potassium, etc., and
is then immersed in a solution of nitrate of silver. Metallic silver
takes the place of potassium, forming insoluble iodide of silver on the
film. The plate is then exposed and the latent image developed by an
aqueous solution of pyrogallic acid, or protosulphate of iron. When
sufficiently developed, the plate is washed, and the image fixed by
dissolving the unacted-upon iodide of silver with a solution of cyanide
of potassium or hyposulphite of soda. This completed the negative or
stencil from which the positives are printed by passing rays of light
through it upon sensitive paper.
_The Ambrotype_ succeeded the Daguerreotype, and was produced by making
a very thin negative by under exposure on glass, using the collodion
process, and, after drying, backing the glass with black asphaltum
varnish or black velvet, causing the dense portions of the negative to
appear white by reflected light, and the transparent portions black.
Such pictures were quickly made, and were much in vogue forty years ago,
but are now obsolete. A modification of the ambrotype, however, still
survives in what is known as the “tin-type” or “ferro-type.” In the
tin-type the collodion picture is made directly upon a very thin iron
plate, covered with black enamel, which both protects the plate from
the action of the chemicals in the bath, and forms the equivalent of the
black background of the ambrotype.
_Silver Printing._--A sheet of paper, previously treated with a solution
of chloride of sodium and dried, is sensitized in an alkaline bath of
nitrate of silver. When the paper is exposed under a negative, the light
through the transparent parts of the negative reduces the silver,
converting the chloride, it is supposed, into a metallic sub-chloride of
silver which becomes dark or black, and constitutes the main portion of
the picture. The image is then fixed by dissolving out the chloride of
silver unaltered by light in a bath of hyposulphite of soda. After
fixation, the image is well washed in several changes of water to
eliminate all traces of the hyposulphite of soda and prevent the
subsequent fading of the darkened portions of the picture and the
yellowing of the whites. If the printed image is immediately fixed, it
will have a red color. To avoid this it is washed first in water and
then immersed in a chloride of gold toning bath and fixed.
_The Platinotype Process_ is one in which potassium chloroplatinite and
ferric oxalate are converted by light into the ferrous state, and
metallic platinum is reduced when in contact with the ferrous oxalate of
potash solution. The unacted upon portions are dissolved out by dilute
hydrochloric acid, leaving a black permanent image. This process is
characterized by simplicity, sensitiveness in action, permanence of
print, and a peculiarly soft and artistic quality in the picture.
British Patent No. 2,011, of 1873, to Willis, is the first disclosure of
the platinotype.
_Carbon Printing_ is a process in which lampblack or other
indestructible pigment is mixed with the chemicals to render the
photograph more stable against fading from the gradual decomposition of
its elements. Mungo Ponton, in 1838, discovered the sensitive quality of
potassium bichromate, which led up to carbon printing. Becquerel and
Poitevin, in Paris, in 1855, were the first to experiment in this
direction, and Fargier, Swan, and Johnson were successors who made
valuable contributions.
_Emulsions._--A photographic emulsion is a viscous liquid, such as
collodion or a solution of gelatine, containing a sensitive silver salt
with which the glass plate is at once coated, instead of coating the
plate with collodion or gelatine, and then immersing it in a sensitizing
bath. The desirability of emulsions was recognized as early as 1850 by
Gustave Le Gray, and in 1853 by Gaudin. Collodion emulsion with bromide
of silver was invented by Sayce and made known in 1864. In 1871 Maddox
published his first notice of gelatine emulsion, and in 1873 the
gelatine emulsions of Burgess were advertised for sale. In 1878 Mr.
Charles Bennett brought out gelatino-bromide emulsion of extreme
sensitiveness, by the application of heat, and from this time gelatine
began to supersede all other organic media.
_Dry Plates_ were a great improvement over the old wet process, with its
tray for baths, its bottles of chemicals, and other accessories.
Especially was this the case with out of door work, which heretofore had
involved the carrying along of much unwieldy and inconvenient
paraphernalia. With the dry plate process only the camera and the plates
were needed, and this step marks the beginning of the spread of the art
among amateurs, and the great snap-shot era of photography, growing into
a distinct movement about the year 1888, has since spread over the
entire world. The first practical dry plate process (collodion-albumen)
was published in 1855 by Dr. J. M. Taupenot, a French scientist.
Russell, in 1862; Sayce, in 1864; Captain Abney, for photographing the
transit of Venus in 1874; Rev. Canon Beechey, of England, in 1875; Prof.
John W. Draper, of the University of New York, and the Eastman Walker
Company, of Rochester, were the chief promoters of dry plate
photography. The practical introduction began about 1862 with the
application of the alkaline developer.
The progress of the photographic art may be approximately noted as
follows:
_Process._ _Time Required._ _Introduced._
Heliography 6 hours’ exposure 1814
Daguerreotype 30 minutes’ exposure 1839
Calotype or Talbotype 3 minutes’ exposure 1841
Collodion process 10 seconds’ exposure 1851
Collodion emulsion (dry plate) 15 seconds’ exposure 1864
Gelatine emulsion (dry plate) 1 second exposure 1878
_Mechanical Development._--The photographic camera is but an adaptation
of the optical principles of the old camera obscura, which has been
credited to various persons, including Roger Bacon in 1297, Baptista
Porta about 1569, and others. The essential elements of the camera
obscura are a dark chamber, having in one end a perforation containing a
lens, and opposite it on the back of the chamber a screen upon which an
image of the object is projected by the lens for the purpose of enabling
it to be directly traced by a pencil. The photographic camera,
introduced by Daguerre in 1839, adds to the camera obscura some means
for adjusting the distance between the lens and the screen on which the
image falls. This was accomplished by making the dark chamber adjustable
in length by forming it in two telescopic sections sliding over each
other, and in later years by the well-known bellows arrangement. A
luminous image of any object placed in front of the lens is thrown in an
inverted position upon the screen, which is of ground glass, to permit
the image to be seen in focusing. When the proper focus on this ground
glass is obtained a sensitive plate is put in the plane of this screen
to receive the image.
[Illustration: FIG. 203.--KODAK.]
It is not possible to trace all the steps of development of the camera
which have brought it to its present perfection. Most of the
improvements have had relation to the lens in correcting chromatic and
spherical aberration, and in shutters for regulating exposure, in stops
for shutting out the oblique rays and holders for the sensitive plate.
The “Iris” shutter, so-called from its resemblance in function to the
iris of the eye, consists of a series of tangentially arranged plates
which open or close a central opening symmetrically from all sides.
The ordinary camera of the photographic artist is too familiar an object
to require special illustration. It has been looked into by the rich and
the poor, and the high and the low, all over the whole world. Between
the traveling outfit, and the “look pleasant, please!” of the
peripatetic artist, and the handsome studios of the cities, it is hard
to find an individual in the civilized world who has not posed before
its lens. Through its agency the great man of the day has found himself
in evidence everywhere; the country maiden has many times experienced
the delicious thrill of satisfied vanity as she posed before it, and the
superstitious savage is paralyzed with fear lest the mysterious thing
should steal his soul.
[Illustration: FIG. 204.--FOLDING KODAK.]
In 1851 the first instantaneous views were made by Mr. Cady and Mr.
Beckers, of New York, and also by Mr. Talbot, who employed as a flash
light a spark from a Leyden jar. In 1864 magnesium light was employed by
Mr. Brothers, of Manchester, for photographic purposes, and about 1876-8
Van der Weyde made use of the electric light for the same purpose.
The _roller slide_, or roll film, was invented by A. J. Melhuish, in
England, in 1854 (British patent No. 1,139, of 1854). The films were,
however, of paper. In 1856 Norris produced sensitized dry films of
collodion or gelatine (British patent No. 2,029, of 1856). In later
years apparatus for utilizing the roll film has been greatly improved
and extensively applied by Eastman, Walker & Co., of Rochester, N. Y.
About 1888 a new thing in the photographic world made its appearance. It
was a little black leather-covered rectangular box, about six inches
long, with a sort of blind eye at one end closed by a cylindrical
shutter, substantially as seen in Fig. 203. This shutter was wound up by
a spring operated by a pull cord. In the back of the box was a film or
ribbon of sensitized paper wound upon one spool, and unwinding therefrom
and winding onto another spool, and being distended as it passed so as
to form a flat surface which was directly in rear of the lens. A thumb
piece or key on the top, and a push button on the side, were the only
suggestions of the operative mechanism within. When the button was
pressed the shutter for an instant passed from in front of the lens, and
as quickly covered it again, but in this brief interval an image had
been flashed upon the sensitive ribbon or film, and a snap-shot picture
was taken. By a simple movement of the thumb piece or key, the receiving
roll was made to take up the exposed section of the sensitive film and
bring another section into the range of the lens, for a repetition of
the operation. This little instrument was slung in a case looking like a
cartridge box, and its sensitive roll was able to receive 100 successive
pictures. When the roll was exhausted, it was removed and developed in a
dark room. The device was placed upon the market by the Eastman Company,
and it was called the “Kodak.” The advertisement of the company, that
“You press the button and we do the rest,” was soon realized to be
founded in fact, and in a short while the great era of snap-shot
photography had set in. To-day this form of camera is a part of the
luggage of every tourist, traveler, scientist, and dilletante. In fact,
it has become the familiar scientific toy of man, woman, and child,
interesting, instructive, and useful to all. In Fig. 204 is shown a
modern form of Kodak, which is made in various sizes and is foldable for
compact and convenient portability.
A very convenient and useful development in films is to be found in the
cartridge system, by which the film may be placed in and removed from
the camera in broad daylight. The film has throughout its length a
backing of black paper which extends far enough beyond the ends of the
film to allow it to be unwound, so far, in making connection with the
roll holder, without exposing the film to light, and also to allow it to
be removed without exposure to light, after all the exposures have been
made.
[Illustration: FIG. 205.--HAND PREMO.]
Among the many other ingenious and useful hand cameras may be mentioned
the “Premo,” made by the Rochester Optical Company, and shown in Fig.
205. The “Premo” is arranged for either snap-shot or time exposure, is
adapted to be either held in the hand or mounted upon a tripod, and is
furnished for use either with glass plates or roll films. In Fig. 206 is
shown the “Premo” for stereoscopic work, in which two pictures are taken
at once, a sufficient distance from each other to produce the effect of
binocular vision and give the appearance of relief when viewed through
the stereoscope. Brett’s British patent No. 1,629, of 1853, appears to
be the earliest description of a stereoscopic camera.
[Illustration: FIG. 206.--STEREOSCOPIC CAMERA.]
There have been 2,000 United States patents granted in photography, most
of which have been taken in the past thirty years, and great efficiency
and detail in both the chemical and mechanical branches of the art have
been obtained.
The useful applications of the art have been numerous and varied.
_Portrait making_ is probably the largest field. This was first
successfully accomplished in 1839 by Professor Morse, of telegraph fame,
working with Prof. John W. Draper, of the University of New York.
_Celestial Photography_ began with Prof. Draper’s photograph of the moon
in March, 1840, and Prof. Bond, of Cambridge, Mass., in 1851. In 1872
Prof. Draper photographed the spectra of the stars, and in 1880-81 the
nebulæ of Orion, and in 1887 the Photographic Congress of Astronomers of
the World, organized in Paris, began the work of photographing the
entire heavens. In late years notable work has been done at the Lick
Observatory by Prof. Holden. In 1861 Mr. Thompson, of Weymouth,
photographed the bottom of the sea, and Prof. O. N. Rood, of Troy, N.
Y., the same year described his application of it to the microscope. In
1871 criminals were ordered to be photographed in England, and in
America the Rogues’ Gallery became an institution in New York as early
as 1857, ambrotypes being first used. In 1876 the Adams Cabinet for
holding and displaying the photos was invented. To-day the New York
collection amounts to nearly 30,000, while that of the National Bureau
of Identification at Chicago approximates 100,000. It is a striking
illustration of the law of compensation that the counterfeiter who
invokes the aid of photography to copy a bank note is, by the same
agency of his photo in the Rogues’ Gallery, identified and convicted.
_Photography in Colors_ has been the goal of artists and scientists in
this field for many years. Robt. Hunt, in England, in 1843, and Edmond
Becquerel, in France, in 1848, made evanescent photographs in colors,
but little progress was made until about the last decade of the
Nineteenth Century. Franz Veress in 1890, F. E. Ives (United States
patent No. 432,530, July 22, 1890), W. Kurtz (United States patent No.
498,396, May 30, 1893), Gabriel Lippmann in 1892 and 1896, Ives in 1892,
M. Lumière in 1893, Dr. Joly in 1895, M. Villedien Chassagne, and Dr.
Adrien, M. Dansac and M. Bennetto, all in 1897, represent active workers
in this field.
[Illustration: FIG. 207.--PANORAM-KODAK.]
Among recent developments of the camera may be mentioned the wide angle
lens, which permits larger images to be made on the plate from small
near-by objects, and the telephotographic camera, which gives a large
image of remote objects, such as an enemy’s fort, and the panorama
camera, which is designed to cover a broad field. For this purpose the
lens is movably mounted for a semi-circular swing, and the image is
flashed across a curved film in the case. The Eastman Panoram-Kodak,
seen in Fig. 207, is an external illustration of this type, and in Fig.
207A is shown a sectional view of another make of panorama camera which
clearly shows the internal construction.
[Illustration: FIG. 207A.--SECTIONAL PLAN OF PANORAMIC CAMERA.]
As allied branches of the photographic art, photo-engraving,
photo-lithographing, and half-tone engraving are important developments
of the Nineteenth Century.
Photo-engraving is a process by means of which photographs may be used
in forming plates from which prints in ink can be taken. The process
depends upon the property possessed by bichromate of potassium, and
other chemicals, of rendering insoluble under the action of light,
gelatine or some similar substance. A picture is thus produced on a
metal plate, and the blank spaces are etched out by acid, leaving the
lines in relief as printing surfaces. When the operation is reversed,
and only the _darks_ are etched in _intaglio_, to be filled with ink, as
in copper-plate engraving, it is called photo-gravure. Mungo Ponton, in
1839, discovered the sensitive quality of a sheet of paper treated with
bichromate of potash. In 1840 Becquerel discovered that the sizing had
an important function, and Fox Talbot, in 1853, discovered and utilized
the insolubility of gelatine exposed to light in presence of bichromate
of potash. In 1854 Paul Pretsch observed that the exposed parts of the
gelatine did not swell in water. One of the first suggestions of
photo-engraving appears in the British patent No. 13,736, of 1851, of
James Palmer. In recent times great perfection in details has been
obtained by Mr. Moss, of the Photo-Engraving Company, and others. The
Albert-type and Woodbury-type are early modifications of this art.
In _photo-lithography_ the photograph is transferred to the stone, and
the latter then used to print from, as in lithography. The operation
consists: 1, in making the photographic negative; 2, printing with it
upon transfer paper coated with gelatine and bichromate of potash: 3,
the transfer paper is then given a coat of insoluble fatty transfer ink
from an inking stone; 4, all ink on surfaces not reached by the light
being on a soluble surface is washed off, leaving the insoluble lines
acted upon by light forming the picture; 5, the washed transfer sheet is
then applied to the stone, and the remaining inked lines of the design
are transferred to the stone; 6, the stone with transferred lines will
now receive ink from the ink rolls on these lines, and repels ink from
all other surfaces, which latter are made repellent by being kept
constantly wet, as in ordinary lithography. The first attempts in this
art were by Dixon, of Jersey City, and Lewis, of Dublin, in 1841, who
used resins. Joseph Dixon, in 1854, was the first to use organic matter
and bichromate of potash upon stone to produce a photo-lithograph. In
1859 J. W. Osborne patented in Australia, and in 1861 in the United
States, a transfer process which gave such great impetus to the art that
he may be considered its founder and chief promotor. His United States
patents are No. 32,668, June 25, 1861, and No. 33,172, August 27, 1861.
[Illustration: FIG. 208.--PHOTOGRAPH GALLERY.]
For photo-lithography only line drawing, type print, or script, without
any smooth shading, can be employed. The most extensive application of
photo-lithography is in the reproduction of the Patent Office drawings,
which amount to about 60,000 sheets weekly. The contracting firm, which
is probably the largest in the world, also prints each week by
photo-lithography 7,000 copies of the _Patent Office Gazette_, of about
165 pages each, including both drawings and claims, and also reproduces
specifications without errors or proof reading, thus saving about 200
per cent. in cost over type setting. This art is also largely employed
for printing maps, and the reproduction of the pages of books by this
process has flooded the stores and news stands with cheap literature.
[Illustration: FIG. 209.--DIAGRAM SHOWING PRODUCTION OF DOT.]
_Half-tone engraving_ enables a photograph to be reproduced on a
printing press, and for faithfulness in reproduction and low cost has
revolutionized the art of illustrating, as nearly all books, magazines,
and newspapers are now illustrated by this process. Before its
introduction it was not possible to reproduce cheaply in printers’ ink
shaded pictures like photographs, brush drawings, paintings, etc.
Half-tone engraving renders it possible to thus print on a press, with
printers’ ink, reproductions of photographs or any shaded picture, in
which the soft shadows fade away in depth to white by an imperceptible
tenuity. It does so by breaking up the soft shadows into minute stipples
which form inkable printing faces in relief, by the interposition of a
fine reticulated screen between the camera lens and the sensitive plate.
This forms a sort of stencil negative through which the copper plate is
etched, which latter is thus converted into a relief plate whose raised
surfaces left by the etching may receive ink and print like an ordinary
relief plate. By making the screen lines very fine (80 to 250 meshes to
the inch), the visible effect of the shading is so far preserved that
the photograph may be reproduced in printers’ ink with but little
depreciation. At first, bolting cloth was used for the screen, but at
present two glass plates, with closely ruled lines, laid crosswise upon
each other, form the screen. A characteristic distinction of half-tone
work is the regularly stippled surface, formed by the stenciling out of
a portion of the picture by the screen, which may be easily seen with
any magnifying glass. It is called half-tone process because half of the
tones or shadows are preserved, the other half being stenciled out. The
use of gauze screens was first described by Fox Talbot in British patent
No. 565, October 29, 1852.
[Illustration: FIG. 210.--TRIMMING FILM.]
In the making of a half-tone negative, the photograph, painting, or wash
drawing which is to be reproduced, is set up in front of the camera,
which is arranged on an inclined runway, as seen in Fig. 208, and an
exposure is made on a plate prepared by the wet collodion process (see
page 304). The shadows of the picture are broken up into stipples or
dots by the interposition of a cross-lined screen arranged in the plate
holder between the lens and the sensitive plate, so that the picture
taken is “half-toned” or stippled. Fig. 209 illustrates the relation of
the parts, in which the picture to be copied is seen on the right, the
camera lens in the middle, and the cross-lined screen on the left in
front of the sensitive plate.
[Illustration: FIG. 211.--STRIPPING FILM.]
[Illustration: FIG. 212.--PRINTING BY ELECTRIC LIGHT.]
The image on the plate is then developed and fixed, and in order to
secure a printed image exactly like the copy as to right and left
position it is necessary to reverse the negative. This is done by
cutting the film square, as seen in Fig. 210, and then peeling it off
the glass, as seen at Fig. 211, and transferring it to another glass
plate in reversed relation. The copper printing plate is produced as
follows: The plate is first polished, as seen at the top of Fig. 213,
and is then sensitized with a solution of organic matter and an alkaline
bichromate. The face of the reversed negative is laid flat against and
in direct contact with the face of the sensitized copper plate, and
tightly held thereto by the screw clamps of the half tone printing
frame. The printing on the sensitized copper face through the stippled
or half-tone negative is then effected either by daylight or by the
electric light. The application of the electric light for this purpose
is shown in Fig. 212. The copper plate is then taken out and subjected
to the three lower operations seen in Fig. 213. It is first developed
under a stream of water from a faucet, seen on the left, and is then
taken in a pair of pliers and held over a gas stove, as seen at the
bottom, to “burn-in” the image, and then placed in a tray containing an
etching bath of chloride of iron seen on the right, by which the copper
is eaten away around the little stipples, and the latter, representing
the half tones of the original picture, are left raised, or in relief,
to form the inkable surfaces of the printing plate. So fine are these
stipples, however, that the picture is to the eye perfectly reproduced.
The several views illustrating this process are made in this way, the
lines of the reticulated screen being 175 to the inch. The plate is next
subjected to the mechanical operation of “routing out” or cutting away
the undesirable portions by a routing machine, seen in Fig. 214. It then
receives further mechanical treatment to correct imperfections and
finish its edges, and is finally mounted upon a block ready for the
printer.
[Illustration: FIG. 213.--TREATMENT OF COPPER PLATE.]
[Illustration: FIG. 214.--ROUTER AT WORK ON HALF-TONE PLATE.]
The most striking application made of photography in recent years is in
the production of so-called moving pictures, in which a series of
photographic figures thrown upon the screen have all the motion of
animated scenes which have been caught and imprisoned by the swiftly
acting and never failing memory of the camera, to be again turned loose
in active play through the Kinetoscope or Biograph. Perhaps the most
valuable contribution to science at the end of the century made by this
art is in surgery, for photographing through opaque bodies by the aid of
the Roentgen rays, but for the latter subjects treatment in separate
chapters must be reserved.