each exposure the paper was reeled off, bringing a fresh surface behind the lens. The spool of exposed paper was removed in a dark room and developed there. From the paper negatives thus obtained, prints were made on the same plan as from the paper negatives made by Fox Talbot. Or by a tedious process the developed gelatine film was removed from the paper support and mounted upon a transparent gelatine sheet, which avoided the grain of the paper. A year later improvements were made in the manufacture of celluloid films, whereby they could be produced in long strips, and these supplanted the paper at first employed. The film of the present day is simply a refinement of the first celluloid rollable film, with the additional improvement that it is wrapped in an extra length of non-actinic opaque paper, allowing the roll to be inserted in and removed from the camera in open daylight, thereby dispensing with the usual dark room. The rollable film is of great utility to tourists and others. The manipulation of the gelatine film requires the use of cool solutions not exceeding a temperature of 80° F., on account of its tendency to soften and dissolve away if too warm. This tendency is checked by the use of alum or formaldehyde.

On the Continent, in the United States and some other countries, the ferrous oxalate developer (first proposed by Carey Lea in 1878) was at first preferred for developing the bromide film either on plates or paper; it is prepared by mixing two chemicals separately. First a saturated solution of neutral oxalate of potassium is made acidified slightly with oxalic acid, then a solution of sulphate of iron 500 grams to 1,000 c.c. of water slightly acidified with sulphuric or acetic acid. Prior to development one part of the iron solution is mingled with six parts of the potash solution, which makes a sherry-colored solution. In this several plates or sheets of bromide paper may be successively developed. In England the pyro ammonia developer was preferred and is prepared by dissolving about four grains of pyrogallic acid in one ounce of water and adding a drop or two of strong ammonia. The negative obtained had a brown and yellow color rendering it a slow printer. In 1882 Herbert Berkley discovered that a small quantity of neutral sodium sulphite added to the pyro ammonia developer retarded the oxidation of the developer and prevented the yellow pryo stain yielding negatives of a bluish black color.

Soon after this, in the United States, prior to 1884, H. J. Newton suggested the use of sodium carbonate (ordinary sal soda) as a subsitute for the alkali ammonia in the pyro developer, which with sodium sulphite made a solution that was particularly adapted to the production of negatives having quick printing qualities. The sodium sulphite prevented the yellow stain, and the carbonate of sodium was more stable than the evaporating ammonia. The proportions of the developer were:

305 c. c. of water. It was found about this time to obtain the best results on plates having had instantaneous exposures, that potassium carbonate as an alkali superseded soda, and this is largely used at the present time, in combination with soda, particularly for the development of shortly-timed plates. In 1889 and since then the new coal tar developing agents were introduced under the name of eikonogen, metol, glycin, ortol, etc. They largely take the place of pyrogallic acid. The fixing agent for dissolving out the creamy unacted-upon film after development is hyposulphite of soda.

The

Printing methods in photography have been as varied and their improvements as great as in the case of the negative. At first prints were made on plain silver chloride paper, and when the ammonia-nitrate was substituted for the plain nitrate, some were made that are not excelled by anything at the present day. desire for greater detail, however, brought into use albuminized paper, which not only came into universal use, but held its sway until comparatively recent times. About a decade ago it was displaced by paper coated with a chloride emulsion, a highly glossy family, generally known as the "aristo." This still continues in use and is largely used in prints intended for process reproduction, on account of the clear rendering of fine details. For truly pictorial work methods which give "mat" or plain paper prints are preferred. The principal, or the most generally employed, are the "carbon" and the "platinum" methods, both introduced in the '60s' though both lay dormant for many years.

ess.

The carbon, probably the best of all printing methods, although platinum is a close second, is more of a mechanical than a chemical procIt uses a paper coated with bichromated gelatine, colored with finely divided carbon or other pigment. This is exposed under a negative, and wherever light has reached the "tissue," as it is called, and just in proportion to the quantity or intensity of that light, the gelatine becomes insoluble. Immersed in warm water, the soluble parts of the tissue (those protected by the opaque or semi-opaque parts of the negative, and consequently the lights of the picture) soften and are washed away. Who first proposed carbon is uncertain. Fargier, in France, was early in the field, but to Swan of England is due the credit of first making it a practical process; although Blair of Perth, Scotland, was the first to recognize the crucial part of it the necessity for exposing through the back of the tissue, or in other words, developing from the side opposite to that which was exposed. Platinum was introduced by Willis of England in 1874, and is based on the fact that a platinum salt is reduced to its metal in the presence of potassium oxalate and a ferrous salt of iron. At first the paper was coated with the ferric salt of iron and exposed to light under the negative, the light changing the ferric to the ferrous salt. Development was effected by a hot solution, a mixture of the oxalate and the platinum salts; but more recently the platinum has been mixed with the iron, and development carried on in a cold solution of potassium oxalate.

During recent years two modifications of the carbon printing process have come into pretty general use, especially among pictorialists; "gum-birchromate" and "ozotype,»

They are simpler than the original method, do not reverse the image, need no transfer and are supposed to give greater control.

In the gum-bichromate process, paper is evenly coated with a suitable mixture of gumarabic, coloring matter and potassium bichromate, and dried. It is printed under a negative in the ordinary way and developed by floating on water of suitable temperature, assisted by brush action, letting the water fall in streams and sometimes mixed with sawdust to assist the removal of color from the lights.

In the ozotype process perfected by Manly, paper is coated with a patented sensitive solution consisting of potassium bichromate and certain other salts, dried and printed as in gum-bichromate; and may then be kept indefinitely. To develop the print the printed paper, with a slightly visible image, is soaked in a solution of hydroquinone acetic acid, copper or iron sulphate, etc., according to the effect desired, and, under the solution, brought into contact with a piece of carbon tissue, or the "plaster" prepared by the patentee. Development takes place in warm water, the coloring matter of the plaster or tissue adhering to the parts of the print acted on by light. The ferro prussiate process (blue print process) was discovered in 1842 by Sir John Herschel.

Not the least important of what may be called the side issues, or secondary applications of photography, are the various productions used in printing. See PHOTO-ENGRAVING.

Photography in natural colors, or, as Sir W. de W. Abney has it, in the colors of nature, has been the dream of many experimenters; but, notwithstanding all that has been done, we are no nearer it than when they began. Color photography, however, that is, photographs having the semblance of the natural colors, has made considerable progress. Becquerel was the first to secure on silver chloride something approaching the colors of the spectrum, but got no further; and to Ducos du Haroun is due the credit for, in 1869, clearly foreshadowing the three methods which include all that has as yet been done in it the superimposing of three-color images, Joly-McDonough colored lines, and Lippmann's interference process. Taking them in the order of their least importance, Lippmann's method is to expose a very thin sensitive film backed by mercury as a reflector, to the colored object. Incident light reflected from the metallic mirror in contact with the film results in interference, and, as the constituents of white light are of varied wave-length, produces in the film a series of planes parallel with its surface, emitting colored light exactly as does the soap-bubble; but the process is difficult, and not likely ever to be more than a scientific curiosity. In the JolyMcDonough method a negative is made in the ordinary way, but with a glass plate with closely ruled colored lines in front of and in contact with the sensitive plate. From the negative so made a positive is printed, and a second or viewing screen with similar colored lines is placed in contact with it, and in exact register with the impressed lines, the result being a picture in the semblance of the natural colors.

An improvement over the screen color lined plate was made in 1906 by August and Louis Lumière of Lyons, France, manufacturers of plates and films, by the introduction of single

5

glass plate coated with a special transparent film, upon which is sprinkled a composite mixture of colored microscopic dust-like starch (potato starch) grains, colored respectively orange, green and violet, there being about 5,000,000 colored grains to the square inch. After the plate is thus coated it is brought under pressure by special mechanical means which flattens out the minute colored starch grains, causing them to merge into each other, giving the appearance, under the microscope, of a mosaic formation. Viewed by transmitted light, the screen appears to have no color.

Upon the screen film thus formed the orthochromatic silver sensitized gelatine emulsion is flowed, and when dry the plate is packed ready for use in the camera, like an ordinary dry plate, except that it is inserted in the plate holder film side down, against a sheet of black surfaced paper, which comes with a box of plates. Thus the glass side of the plate is next to the lens.

A special yellow colored filter intended to absorb a portion of the blue rays of light is interposed in the camera between the lens and the plate. The light from the object to be photographed, after passing through the lens and color filter, first impinges upon the glass side of the color sensitive plate, then penetrates the screen film and lastly acts upon the back of the sensitized film, affecting the film automatically in proportion as the color particles of the screen film transmit the colors of the object photographed to the sensitized film.

The exposure of the plate in the camera for any given stop or diaphragm is usually about 50 times longer than for an ordinary fast plate. The developer used is of the metol-quinone type, having liquid ammonia as an accelerator,

The

After exposure, the developer (at a temperature of between 60° to 65° F.) is applied to the plate (placed in a tray) preferably in a room that is perfectly dark, for two and a half minutes. It is then poured off, the plate rinsed with water, then a reversing solution (permanganate of potassium) is applied (under a bright light) for three minutes which dissolves away the black reduced silver negative image, converting the same into a positive image. plate is next rinsed under the tap and the same developer is again used in the tray a second time in bright daylight, which converts the unreduced bromide of silver (or what would represent the shadows in the original negative image) into dark metallic silver, and thus completes the manipulation required to make the transparency. The plate, after removal from the developer, is washed under the tap for a brief period, and, on viewing the same by transmitted daylight, a beautifully colored transparency, possessing all the gradations of the color of the original, is observed.

The operation is nearly as rapid as that of making an ordinary tintype. The finished picture is termed an "autochrome," since its coloring is automatically obtained.

The "three-color" method is the most important, as it has the greatest commercial possibilities, and gives the most varied and most satisfactory results. Although Collen, in 1865, was probably the first to suggest the method, and du Haroun, in 1869, outlined it clearly, they and those that followed them were on the wrong track, working on the theory of Brewster, which

never could lead to success, instead of that of
Young, Helmholtz and Maxwell, now univer-
sally accepted. The first to recognize this was
Fred E. Ives of Philadelphia, and to him more
than to any other, or indeed to all the others
together, are we indebted for the great progress
that has already been made. The first experi-
mental half-tone three-color plates and prints
were made in 1881.

In 1910 Mr. Ives introduced an improved
system of color photography by which dupli-
cate color photographs on a transparent film
were obtained. In a specially constructed
camera three sensitized dry plates, two of
which were sensitive to red and green rays of
light and one to blue rays, were exposed
simultaneously, after passing through the
single camera lens, between which and the
plates was interposed a yellow-orange trans-
parent light filter or screen for the purpose
of reducing the strength of the blue rays.

The three sensitive plates were enclosed in
a single plate holder, specially designed, so
arranged that two of the plates were placed
film sides in contact and a third, a trifle smaller
in size, with the film side against the glass side
of one of the two plates in film contact. The
three plates were secured together at one edge
by a flexible paper hinge and the whole was
named a "tripack."

To make an exposure in the camera (after
focusing) the holder (containing the three
plates) is inserted like an ordinary plate holder
in the usual way. On withdrawing the plate
holder slide, the smaller plate in the holder
falls out by gravity and rests on the bottom of
the camera in a horizontal position, but the
other two plates are held in the holder in a
vertical position. After this operation a second
yellow tinted transparent plate on the interior
of the camera was dropped by a lever on the
exterior, downward from the roof of the
camera in front of the tripack plate holder
at an angle of 45 degrees, the top of
the plate being nearer the lens than the
bottom, and formed a transparent reflec-
tion since its location was over the smaller
sensitive plate lying on the bottom interior of
the camera. Part of the light from the colored
object, after passing through the camera lens
and the yellow absorption filter, is reflected
downward by the transparent reflector upon the
two horizontal blue sensitive plates, while the
rest of the light continues on in a horizontal
direction, striking the underside first of the red
sensitive plate film, passing through the same
and acting upon the face of the yellow-green
sensitive plate film in contact therewith. Thus
three plates are in focus and exposed at one
time. After exposure the angular filter screen
plate is carried back to the camera roof, a lever
on the outside raises the horizontal smaller
plate into the plate holder till the plate holder
slide is pushed in. The three exposed plates
are each marked to indicate their respective
color sensitiveness and are then developed as a
unit in a tank developer for a specified time.
Each image is distinct and sharp and exactly
the same size. The duplicate positives are made
from the three negatives upon a transparent
film sensitized with a gelatine bichromated solu-
tion, printed in a printing frame in sunlight all
three at one time, fixed in warm water. Where
the light has acted, renders film absorbent to

color. Each respective film picture is next
dipped in its blue, red and yellow dye solution,
washed, and when dry are clamped together so
that images perfectly match between two pieces
of glass, with the result that a beautifully
brilliant colored very transparent picture, true
to nature, is obtained. See COLOR PHOTOGRAPHY.
A serious objection to silver-bromide, the
sensitive salt in the ordinary photographic plate,
is the fact that it is so much more sensitive to
the blue-violet than to the green and red of the
spectrum as to give a very false rendering of
color values or luminosities, the darker colors
showing as the lighter and vice-versa. Dr. W.
H. Vogel was, perhaps, the first to find that the
addition of certain dyes to the emulsion tended,
to a large extent at least, to equalize the sensi-
tiveness; and to plates prepared with such an
emulsion or soaked in the dyes after prepara-
tion, he gave the name of orthochromatic or
isochromatic. But even the most evenly sensi-
tive of such plates are still more sensitive to the
blue-violet than to the red and green and for
the best results the true rendering of color
luminosity, it is desirable to employ color filters
or screens that will absorb the excess of blue-
violet.

The beginning of the moving picture idea
was early in 1878, prior to the introduction of
the present rapid dry plate, by E. J. Muybridge,
a photographer in San Francisco, Cal., who
photographed the movements of a racing horse.
owned by Leland Stanford by having a series
of individual cameras placed in a row about one
foot apart, the shutters of which were electri-
cally operated automatically, as the horse moved
forward. This first experiment proved that a
trotter's feet are entirely off the ground to-
gether twice during the making of a stride. In
1896 Edison and others perfected the moving
film system in combination with a shutter. See
MOVING PICTURES.

In scientific investigation, photography has
been remarkably helpful. It has taken the place
of manual labor in record-keeping, measured
the velocity of flying bullets, shown the true
positions of animals in motion and created the
"new astronomy." As early as 1840, Draper
had made photographs of the moon; in 1857
De la Rue established heliographic observation
of sun spots, a work begun at Kew and con-
tinued at Greenwich till the present time. Em-
ployed in every solar eclipse since 1860, photog-
raphy has told us all that we know of the
corona and chromosphere; it has revealed the
mystery of the hitherto puzzling nebulæ, and,
by virtue of the fact that the light action is
cumulative, has told of the existence of stars
so far away as to be beyond the ken of the
human eye, aided by the most powerful telescope
yet made. At the Astro-photographic Con-
gress that met in Paris in 1887, it was resolved
to make a photographic survey and star-map
of the heavens and at present there are 18
telescopes, each of 13-inch aperture, engaged
in that work in various parts of the world,
while in most of the more important observa-
tories protography and the spectroscope are
joined together and made daily, or rather
nightly, to tell us more and more of the mystery
of the heavens.

As an educational adjunct, photography has
played an important part. The projection lan-
tern finds a place in every well-equipped lecture-

room, and the photographic lantern-slide lends itself equally to the teaching of science and the illustration of travel. The beauty and accuracy of the photographic lantern-slide and the ease with which it is made make it equally available to the college professor and the itinerant lecturer, enabling the one to show to a whole class what otherwise would require to be handled by the members one by one; and giving to the other an opportunity of making a comfortable living and in some cases amassing a fortune, by amusing and instructing the popular audience. Hardly less important, although much less popular, is the enlarging of small objects, "photomicrography." In bacteriology, histology, etc., its importance can hardly be overrated, affording, as it does, illustrations in works dealing with such subjects that are without a suspicion of the imperfections of draftsmanship and showing, as they do, when orthochromatic plates are employed, the different luminosities of the various stains.

Nor is photography less important from a social point of view. While it displaced miniature painting, a style that only the rich could enjoy, it gave a better likeness of loved ones equally available to rich and poor. It has given us correct instead of fancy or distorted views of the manners, customs and scenery of distant lands; enabled the cottager to decorate his walls with better pictures than were available to his richer neighbor previous to its advent and given a new interest to periodical literature by the low cost and excellent quality of its illustrations. Not less wonderful has been its influence commercially. It has created new branches of trade and manufacture and largely increased many that were in existence before, furnishing well-paid work to hundreds of thousands of both men and women. The glassmaker and the optician have wrought together till they have given us lenses as nearly perfect as we can hope to see; the chemist has given us new material and improved the old, building factories for the manufacture of some by the ton that, previous to the advent of photography, were only known as curiosities of the laboratory; while the camera-maker has so exercised his ingenuity as to give us cameras of perfect workmanship and almost automatic in their action. In 1914 there were 87 establishments in the United States manufacturing photographic apparatus, of which 21 made almost exclusively cameras and 21 motion-picture machines. In the same year there were reported 59 factories making photographic materials of the gross value of $4,273,000. This latter industry employed 6,658 people and made gross products of $34,768,000.

Bibliography. Harrison, History of Photography (1887); _Wall, 'Dictionary of Photography) (1897); Payne, 'Wet Collodion Process' (1907); Derr, 'Photography for Students' (1906); Holme, Colour Photography) (1908); Cassell, Cyclopedia of Photography (1911); Jones, Photography of To-Day) (1912); Hance, Commercial Photography) (1914); Roebuck, 'The Science and Practice of Photography) (1918); files of The Camera, Photographic Journal, etc. See PHOTO-ENGRAVING. JOHN NICOL, Ph.D., FREDERICK C. BEACH, Ph.B. PHOTOGRAPHY IN COLOR. COLOR PHOTOGRAPHY.

See

7

a

PHOTOGRAVURE, fō"tō-grå-vür', process of engraving in which, by the aid of photography, subjects are reproduced as plates suited for printing in a copper-plate press. The process known as heliogravure is essentially the same. See PHOTO-ENGRAVING.

PHOTO-GLYPTOGRAPHY, that department of photo-engraving in which the plates are in intaglio. See PHOTO-ENGRAVING.

PHOTO-HELIOGRAPH, an instrument for observing transits of Venus and other solar phenomena, consisting of a telescope mounted for photography on an equatorial stand and moved by suitable clockwork.

PHOTO-LITHOGRAPHY, a method of producing by photographic means designs upon stones or zinc or aluminum plates, from which impressions may be obtained by lithographic process. The first requisite for the production of a good result by this process is a suitable original. The drawing should be made with perfectly black lines throughout, no matter how thin the lines are; the scale of reduction should not be too great; the best proportion is obtained when the drawings are made about onethird larger than the required block; the paper used should be white and smooth in texture.

The negative for a line-block is made preferably by the wet-plate or collodion process, because of the facility with which these plates can be intensified and the clearness of the lines. Asser of Amsterdam was the first to put photolithography to practical use, but probably it is to Osborne of Melbourne that we are indebted for the modifications which made it the process now employed by every map-maker in the world. A sheet of suitable paper is coated by floating on a solution of bichromated gelatine or albumen; printed under a negative and inked either by a roller or, better still, by spreading the ink evenly and passing the paper through the press once or twice as if drawing a proof. The inked sheet is then laid face down on warm water if gelatine has been employed and cold if albumen. The gelatine, where light has not affected it, swells and dissolves, leaving ink only where light has acted, the parts representing the dark lines of the original. A spray with water or even a slight wash with a sponge makes it ready, after drying, for transferring to the stone or plate and the quality of the work will depend on the care given to the preparation of the transfer.

Photo-lithography is the principle used in offset printing. See OFFSET-PRESS under PRINTING PRESSES; also PHOTO-ENGRAVING. PHOTOMETER, an instrument intended to indicate relative quantities of light, as in a cloudy or bright day, or to enable two lightgiving bodies to be compared. A photometer in common use was invented by Bunsen; it consists of a screen of thin paper moistened with a solution of spermaceti in turpentine, except a spot in the centre. This screen being placed on a stand at a fixed distance from a source of light of constant intensity, the ungreased spot appears darker than the greased part. One of the lights to be compared is then placed in front of the screen and adjusted at a distance such that the ungreased spot is illuminated as much as the rest of the screen. A similar experiment being made with the other light to be compared, the intensities of the two

are to one another in the proportion of the squares of the distances from the screen at which the lights must be placed in order to cause the disappearance of the ungreased spot. Other photometers depend upon sensitizing a paper which is darkened by exposure; and in variations in the resistance of a selenium cell. The art of measuring the intensity of a source of light is termed photometry.

PHOTOMETRY is the art of comparing the intensity of a source of light with that of another source which is taken as a standard. The possibility of making such comparisons depends upon the power of determining by means of the eye when two neighboring fields of view, illuminated respectively by the two sources in question, are equally bright. The sensitiveness of the eye to inequalities of brightness does not greatly exceed 1 per cent, even under the best conditions; and since it frequently falls below that value from fatigue and from various other causes, numerous attempts have been made to find photometric methods which are independent of this organ, but thus far without much

success.

Photometers. Any instrument for the measurement of the intensity of a source of light is termed photometer. Since all attempts to substitute for the eye such instruments as the thermobile, the bolometer and the selenium cell have, for the ordinary purposes of photometry, led to unsatisfactory results, all existing photometers which have come into general use are based upon the above-mentioned power of the eye. The earliest form, which was originally described by Bouguer, was invented early in the 18th century. It consisted of a screen AB (Fig. 1), illuminated by the sources of light S and the intensities of which were to be compared. The partition PC prevented the light of S from falling upon BC and that of s from reaching AC. The distance of the two sources from the screen was adjusted until the illumination of AC appeared to the eye to be equal to that of BC.

Since the illumination produced by a source of light is inversely as the square of its dis

from one of the sources exclusively. When these shadows are equally bright the distances of S and s from the shadows which they illuminate determine the relative intensity of the two sources.

Ritchie (1826) introduced a new principle into photometry. He placed the two lights to be compared at the ends of a track or bar along which a box containing two mirrors M,M' (Fig. 3) could be moved, until the rays from S, re

flected at M to the left half of the screen AB gave an illumination equal to that from the rays from s reflected by M' to the other half of the screen. This screen was of some translucent material, usually paper.

Bunsen (1841) substituted for the Ritchie screen a sheet of unsized paper the central, usually circular, portion of which had been rendered translucent by the application of oil or of melted paraffin. The paper when placed between two sources of light, the plane of the paper perpendicular to the incident rays on either side, affords a very simple and convenient means of determining when the illumination from the two sides is equal. When subjected to unequal illumination from the two sides the translucent portion of the face toward the brighter source appears dark, the unoiled portion bright. On the other face the reverse is true (see Fig. 4). As the paper is moved away from the brighter source and toward the weaker an interchange in the appearance of the two surfaces occurs and there is a neutral position in which both appear alike and in which it is scarcely possible to distinguish the translucent portions. When this position has been found the relative intensities of the two sources may be calculated from the law of inverse squares. To facilitate the observations the bar or track upon which the paper screen is mounted is divided into a convenient number of equal parts.

The sheet of paraffined paper, technically known as the Bunsen disc, is usually mounted in a wooden box with blackened walls (the photometer carriage), which slides or rolls along the track between the sources of light. Two small mirrors (M and M', Fig. 5), mounted obliquely within the box, enable the operator to observe simultaneously the two faces of the paper (0.0.).

In practice the paper is frequently used with an unoiled central disc, the remainder of the surface being rendered translucent by treatment with oil or paraffin. To avoid the use of oil or paraffin which gives a surface which soon becomes soiled from dust in the air, two similar paper screens are sometimes employed. Identical portions, in form either a disc or a star, are cut out from the centre of each. A sheet of tissue paper placed between the two then affords a translucent region which takes the place of the oiled paper of Bunsen's original device.

The various forms of photometer already described and all others which depend upon the power of the eye to detect slight inequalities of illumination are essentially of equal sensitiveness. They are all limited by the sensitiveness of the eye and approach the maximum degree of delicacy as we fulfil more and more nearly the conditions under which the eye can be used to the best advantage. All forms of the Ritchie and Bunsen photometers, of each of which many modifications have been devised, are, however,