WOOD'S DIFFRACTION GRATING PRO CESS This is practically a modification of the three colour process in which diffraction gratings ruled with a varying number of lines to the inch are used instead of the three stained filters, and the principle of the process can be easily grasped from the following explanation. In the diagram s is an intense source of light ; between it and a lens 1.„ is a diffraction grating G. There will be obtained not only the central white image, as indicated, but also on both sides a spectrum, and for the moment only consider the one spec trum, S '. Now, if we make an aperture in a card at the point where red falls, and place the eye there, we shall see red only. If we replace the grating by one with finer rulings, then we shall find the green fall in the place of the red, and if we use a finer grating still, we should find there the blue. Now, if we put the two first gratings in front of the lens and overlapping one another, the red and green would fall on the aperture in the screen, and this would give us the sensation of yellow. If the third grating be added, then we should have red, green and blue light reaching the eye, and the result would be white.
The gratings used should have z,000, 2,400 and 2,700 lines to the inch respectively for red, green and blue light to fall in the same spot. To produce a picture in colours, three negatives are taken in the usual manner through red, green and blue-violet screens as in ordinary three-colour work, and from these positives are made. A sheet of thin patent plate should be flowed over with the following : Gelatine . 384 grs. 40 g.
Potassium bichromate (sat. sol.) . . mins. 16-24 ccs.
Distilled water to . . 20 oz. i,000 „ Filter whilst warm and allow the glass to drain for about io seconds, and then set on a level slab to dry. The sensitised glass is placed in
contact with one of the three positives. The two plates should be held up in front of a lamp and register marks made on the glass surface of the sensitised plate, using minute ink dots and some prominent object that appears in all three pictures. Corresponding ink dots should now be made on the film side of the sensitised plate and the others rubbed off.
Then the grating with the coarsest ruling, which furnishes the red light, should be placed on the sensitised plate and on this the positive of the red light sensation. The lines of the grating should be vertical and the ink dots made to carefully register with the object in the positive. Expose the plates to sunlight for about 3o seconds, gripping them tightly in the fingers and holding them perpendicularly to the sun's rays. It is advisable to do this in a darkened room, using only a narrow beam of sun light. The exposure will be about 3o seconds. Now remove the positive and grating, and sub stitute those for the green picture and repeat the operation, and finally, repeat the operation for the blue picture. Or preferably wash the plate in warm water after exposure through the red and green positives. The blue picture can then be made on a separate plate, placing the positive film out so as to reverse it, and after exposure, washing the plate, and then super imposing and building up.
The apparatus for viewing these pictures is a lens cut square like a reading glass mounted on a light frame, provided with a black screen perforated with an eyehole.