POPULAR SCIENCE

Otago Witness, Issue 3455, 1 June 1920, Page 42

POPULAR SCIENCE

PRINCIPLES OF COLOUR PHOTOQRAPHY.

. The fourth of Dr Jack's series of popular scientific lectures was delivered to another crowded audience in the chemistry lecture room on the 24th ult. After revising- _ a little of the ground covered in previous lectures, the - lecturer went on to deal with the production, of colour by adding colours together. Any colour at all can be produced by making the correct combinations of violet, green, and red. Speaking of how the eye detected colour, ho stated that at the back of the eye there are three seta of nerves, correspondine to each of the three primary colours—violet, red, and green. If we looked at a red object the red sensitive nerves became active and similarlv with the other primary colours. • If we looked at yellow, which is made up of green and red, then the green and red nerves became active at the same time, and gave the sensation we called yellow. If the sensations of red and green were made to follow quickly after each other on the retina we got the sensation of yellow, because the retina had not forgotten the first sensation before the second came. It was on this property of retaining the image for an appreciable time that the kinematograph was based* A number of fascinating experiments were employed to illustrate the addition of colours. This principle, he said, had been used in the kinemacolour pictures, in one form of which two sets of pictures were taken—one through a red screen and one through a green screen. The scheme was not perfect, because they did not have the violet as well, and there were certain difficulties in the way of using violet. In animals that were not mammals the retina of the eye was differently constituted from that oilman. In the tortoise, for instance, the retina contained tiny globules of violet, green, and red, and in the domestic fowl there were globules of green, yellow, and red. These coloured globules gave the same effect- as the specialised nerves" in the human ey-3. Proceeding to the subject of colour photography, the lecturer explained the prepara*tion of the photcgrax>hic plate with green, blue, and red starch grains. Behind that was the sensitive film. Then green light, for instance, acted only on the part of the sensitive film immediately behind-the green starch grains. Similarly, vellow passed only through the green and red grains, and similarly for the other colours. There remained the problem of getting rid of the silver deposited by the light on the developer, and once that was done, by a special solution, the plate could be developed and fixed, and it would be found that the light passed through it would give the natural colours. With aid of a' specially-prepared screen and slides and other apparatus supplied by Mr Earland, an enthusiast in colour photography, the lecturer was able to display a series of beautiful examples of colour photography. Dr Jack next explained and illustrated stationary waves, showing that a sensitive plate was acted on only at the points where the light waves were in motioa. The application of these facts to another form of colour photography brought the lecture to a close.