There may be some relation between the monochromatic patches and the discs in the outer segments of the cones. These are about sixteen in number in the guinea-pig. As in photography, the intensity of the light is a very important factor in vision. With colours of moderate intensity, the periphery of the retina is found to be colour-blind, but this apparent colour-blindness disappears when more intense lights are used. A person may have shortening of the spectrum with light of moderate intensity, but when the light is increased be able to recognise the spectrum to its normal limit. The change in steepness of gradation, according to the intensity of the light, is well known to photographers. The Purkinje effect, a change in maximum sensitiveness of the eye according to the intensity of the light, is, in my opinion, a photo-chemical effect. I find that the Purkinje effect is found for small portions of the retina if a black object has been situated in the corresponding part of the field of vision. The yellow pigment which is found in the yellow spot probably acts like the yellow screen in photography, which, by absorbing the blue and violet rays of the atmosphere, renders visible that which would otherwise be invisible. This is further borne out by the fact that hunters in India are able to hunt later in the day than usual by using spectacles glazed with golden yellow glass. THE FACTS OF COLOUR-BLINDNESS When we consider the path along which a visual impulse has to pass, and that each cell has probably some special function in connection with that impulse, it is not surprising that we meet with a large number of different defects of colour-perception and light-perception. Defects of light-perception are quite distinct from defects of colour-perception. 1. _Defects of light-perception._--The person having the defect is placed in a similar position to a normal-sighted person with those particular rays removed or reduced to the same intensity. Defects of light-perception may be caused by absorption or by some defect in the visual purple or cerebro-retinal apparatus. The chief defect of light-perception which is found is shortening of the red or the violet end of the spectrum. Let us consider the influence of a shortened spectrum upon colour-vision. The first evident fact is that bodies reflecting only light, the rays of which occupy the missing portion of the spectrum, appear black.