The advance of photography : its history and modern applications (1911)

278 THE ADVANCE OF PHOTOGRAPHY the spectrum except the red, that is, green and violet. The negative obtained with the green filter will similarly be used for the block intended for printing with the magenta ink, while the positive block made from the negative photographed when employing the blue filter will be used to obtain the yellow. The reason for this is as follows. The light received by the plate sensitized for red light is obtained through a red filter, hence only the red rays of light proceeding from the copy produce any deposit on the plate. When this plate is used for printing on the prepared copper, those portions of the glue which are under the parts of the negative affected by red rays will be uninfluenced by exposure to light, and will be dissolved away when the plate is developed. Hence the image obtained is not one of the red part of the object, but of those parts which emit other rays than the red, viz., the green and violet. When all three blocks are ready the printing must be first of all performed with the yellow ink, next with the red, and lastly with the blue. The six accompanying illustrations will show how the picture appears after each of these operations, and will also make clear that very beautiful work can be produced by the use of inks of three colours only. Collotype. — One other method of producing very beautiful pictures, but without the aid of the half-tone screen, must be briefly mentioned : this process is known as Collotype. Negatives are prepared from nature as previously described, but without the half-tone screen, and then prints are made from these upon films of bichromated gelatine which have been placed upon plate glass and dried. Such a film is found to have the abilit}r to take up ink just in proportion to the action of the light upon it during the exposure. The negatives for this kind of work must possess well-marked gradations from the deepest shadows