PHOTOGRAVURE
X478
PHOTOSYNTAX
by this method has thus far been confined to the laboratory, the conditions for success not yet being quite fully understood. Probably not more than half a dozen people in the world have succeeded in getting satisfactory results, owing to the experimental difficulties.
Of the indirect processes there are a number which have been developed far enough to be considered commercial successes. These processes are all based on the principle that any color can be imitated by a mixture of the three primary colors, red, green and blue. The most beautiful results have been obtained by Mr. Ives of Philadelphia, one of the pioneers in ^ this line of work, whose process consists, briefly, in taking three negatives of the subject through red, green and blue glasses. These glasses prevent light of any other color than that which they are designed to transmit from getting at the plate; consequently each negative is a record of the distribution of one of the three primary colors in the original. From these negatives three transparencies on glass are printed, which, when thrown superimposed on a screen by means of three lanterns furnished with colored glasses similar to the ones used in taking the pictures, combine to form a very perfect reproduction of the original.
For exhibiting the pictures without the lanterns Mr, Ives has devised an instrument which he has named the kromskop, in which the colored images are combined by reflectors. The pictures are stereoscopic as well, and the result is a reproduction so perfect that .it is almost impossible to believe that we are not looking at the object itself, for it stands out in full perspective with every color perfect.
Another process, which was perfected independently and almost simultaneously by Dr. Joly of Edinburgh and Mr. Mc-Donough of Chicago has been recently put upon a commercial basis. In this process the negative is taken on a plate in front of which is placed a screen ruled with very fine lines in red, green and blue ink, the colors following each other in succession across the screen. This screen breaks up the picture into linear strips, any one of which may be regarded as a record of one of the three primary colors along that portion of the picture. From the negative obtained in this way a positive is printed on glass, which, when mounted in contact with a similar tricolor ruled screen, reproduces the colors of the original. The objection to this method is the obtrusiveness of the lines, especially when the pictures are projected. The colors are much weaker and less faithfully reproduced than by the kromskop method.
Another method, devised originally by Prof. N. W. Wood and recently improved and perfected by Mr. Ives and his son, em-
ploys the diffraction grating as the source of color in the picture. The pictures can be duplicated by a purely mechanical process, but are colorless except, when examined with a special viewing apparatus. Results have been obtained by this method very nearly if not quite equal to those yielded by the kromskop. The most recent process, and the only one which appears to have been a commercial success, is the recently devised starch-grain process invented by the Lumiere brothers of Lyons. It is a modification of the Joly process, the colored lines being replaced by stained granules of potato-starch. The plates are not very difficult to operate, and the results are highly satisfactory, though by no means equalling those yielded by the kromskop method. R. W. WOOD.
Pho'tograv'ure. The expense of photogravure work greatly limits its use mostly to high-class bookwork. Large pictures, however, are produced by it which rival the finest steel engraving in finish and delicacy. The photographs can be reproduced, but the process is largely employed for obtaining engravings, such as copies of celebrated pictures. The process is so nearly perfect that any touch of the painter's brush is clearly seen in the copy. We here give one of the methods. (See PHOTOGRAPHY for sensitized plates, negative etc.) A gelatine relief is obtained by exposing bichromated gelatine to the action of 1'ght beneath a negative. The gelatine is mixed with a quantity of black lead in more or less granular form. This causes the relief to have a surface which is granular in character, and also makes it a conducting one for electricity. Put now into an electrotype bath; it will soon be covered with a deposit of copper. A copper printing-plate is thus made, from which pictures are printed.
Pho'tophone, a curious instrument invented by Graham Bell in 1880. It is based upon the fact, discovered by J. E. May hew in 1873, that the resistance of annealed selenium is less in sunlight than in the dark. Using a piece of thin, silvered glass on a diaphragm, Bell was able, by means of the human voice, so to deflect a pencil of snnlight that, with each pulsation of the voice, a distant piece of selenium was alternately illuminated and left in the dark, This piece of selenium was in the circuit of a telephone receiver and a battery. At each illumination a sudden increase of current passed through the receiver, because the resistance of the selenium was diminished. One with his ear at the receiver,, therefore, heard every pulsation of the voice that deflected the sunbeam upon the selenium. Such an instrument he called a photophone.
Photosyntax (fo'td-stn'taks), a term formerly used for photosynthesis.
