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

246 THE ADVANCE OF PHOTOGRAPHY film. When the positives are dry they are coated with a* bichromated gelatine and allowed to dry in a subdued light. Three diffraction gratings, 2000 lines to 1 inch for red,. 2400 lines per inch for green, and 2750 for blueviolet, are placed over these positives and an exposure to the sun made for thirty seconds. When the plates are afterwards, washed in warm water diffraction gratings of great brilliancy are formed directly on the film surface. Three sheets of thin glass sensitized with bichromated gelatine are placed under the three positives and prints, taken from them. The portions of each plate on which the light has acted bear the impression of the corresponding grating, and this impression varies in distinctness according to the density of the different parts of the positives. When these three plates so obtained from the positives are superimposed ,. placed in front of a lens and illuminated by a narrow source of light, a correctly coloured picture is seen if the eye is placed in the correct position. This method, however, did not allow of perfect registration of thethree slides. WTood found that it was not necessary to use three separate plates, but that the three gratings could be obtained on a single film. Thus the final process becomes an exposure, through the transparency made from the negative obtained with the red screen, of a piece of glass coated with bichromated gelatine, over which is laid a grating of 2000 lines to the inch. A second exposure is made of the same bichromated film through a grating 2400 lines per inch and the transparency obtained with green negative ; finally a third exposure is made with grating 2750 lines and blueviolet transparency. When thisbichromated plate has been washed and dried it becomes the coloured photograph. The reason for this is that where reds occur in the original only spacings of the red