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THE CHEMICAL ACTION OF LIGHT 71
dimensions, while that of the plates has a diameter of 5-9 fi.
Although it can quite easily be shown that one would expect the final degree of blackening to be greater with fine-grained haloid for equal percentages of haloid in the emulsion, and it is further known that the degree of blackening produced by exposure to light attained with chloride is greater than that attained with bromide, while the developable image with bromide is the denser, the complete solution of the problem remains yet to be given.
Action of light on the visual purple. — No account of the chemical action of light would be complete without some reference to the way in which the visual purple — a substance present in the outer limbs of the rods of the retina — is affected when exposed to light.
Kiihne carried out a number of experiments upon this purple, and obtained many important results. He found that it is bleached most rapidly by the greenish yellow rays of the spectrum, then by the green, and least of all by the red. When exposed to ordinary light it becomes yellow, and then white. By means of this purple, it is possible to obtain definite photographs on the retina of an excised eye, and these photographs can be fixed.
Dr Edridge-Green * has based his theory of colour vision upon the action of the visual purple. Briefly stated, his views are as follows : — Light falling upon the retina liberates the visual purple from the rods, and this then diffuses into the rod and cone layer of the retina. Light next causes its decomposition, a change in electrical conditions probably resulting. In this manner the ends of the cones are stimulated, and a visual impulse is set up which is conveyed through the optic nerve fibres to the brain. He assumes that the action produced by light varies according to its wave length just as takes place in other photo-chemical effects.
1 For full account of his theory see "Colour Blindness," International Scientific Series.