Colour Photography

The production of the negatives can be accomplished by three different methods. For stationary objects where an interval be tween the exposures is of small importance, it is convenient to fit the filters and sensitive plates into a sliding back attached to an ordinary camera. Exposures are then made through the three filters in succession either upon a single panchromatic plate or on three separate plates.

For the photography of moving objects, it is, of course, im possible to use successive exposures, and a great number of cam eras embodying beam splitting methods either by means of prisms or by reflectors have been designed. The chromoscope already re ferred to as a "viewing" instrument can be converted into a cam era in which mirrors are used for separating the three beams. Other cameras use elaborate combinations of prisms for the same purpose. In such cameras it is often convenient to use separate plates, since it is rather difficult to arrange for the three images to fall upon a single surface.

A system invented originally by du Hauron is to have the sensitive surfaces superposed on each other and to build up a plate or film pack; thus, in front, there may be placed a slow, very transparent yellow-dyed, blue-sensitive film ; behind this, a moderately transparent film sensitive to green and dyed red or with a red filter attached to it ; behind this again, a red-sensitive film. In this way, the blue record is made on the first film, the green record on the second, and the red on the third. A con siderable number of patents have been taken out for variants of this method.

The Subtractive Process.

The subtractive process, which was described originally by du Hauron, depends upon the produc tion of colours by subtractive absorption instead of by the addi tion of the colours to each other in projection. Suppose that over a sheet of yellow gelatine, which will absorb the blue light, is put a sheet of magenta gelatine, which will absorb the green light ; then, since the yellow absorbs the blue, light, and the magenta the green light, only the red light is transmitted; a red image can thus be obtained either by projecting it through a red filter or by putting a magenta image on the top of a yellow one. In the same way a green image can be obtained by putting a blue-green one on the top of a yellow one, when the yellow will cut the blue out of the blue-green and leave only green.

In working the subtractive process, the three negatives are printed in coloured dyes, the picture taken through the red filter being printed on gelatine dyed blue-green, the one taken through the green filter on gelatine dyed magenta, and the one taken through the blue filter on gelatine that was dyed yellow. Now, if the three are cemented together in register, the resulting trans parent colour picture will reproduce the colours of the original subject and will be a transparency which can be viewed in the hand, or examined in front of an artificial light, or projected in a lantern.

Printing Methods.

Subtractive methods have naturally at tracted the most attention for giving prints on paper.

The carbon process was used by du Hauron, the negatives being printed upon carbon tissue containing a pigment of colour com plementary to that of the filter. After development, the three images are transferred to a temporary support, and then re transferred to a permanent support in accurate register. This method gives excellent results but requires considerable skill.

Very good results have also been obtained by the use of a process in which gelatine containing transparent dye was coated on thin film base. After sensitising with bichromate, this can be printed through the back and developed with hot water, so that a coloured relief image is obtained on the film. The three images can then be transferred in superposition on to a paper support without the use of double transfer.

A modification of the carbon process, suggested by T. Manly under the name of "Ozobrome" and later revived by H. F. Farmer as the "Carbro" process, is also used for colour work. In this a silver image is squeegeed into contact with pigmented gelatine saturated with bleaching solutions whose reduction products harden gelatine. These chemicals migrate into the silver image film, and the reduction products return to the tissue, rendering it insoluble in ratio to the amount of metallic silver present in the image. The tissue is then treated exactly as in the carbon process. Bromoil transfers are also used by some artistic workers.