§ 6. Total Colour-Blindness.—The extreme margin of the retina is totally colourblind. Let the eye be fixed upon an object immediately in front of it, and let someone gradually introduce an unknown coloured object into the field of view from one side. On its first entrance into the field of view, the object will appear white, grey, or black. Its colour will only become recognisable as it approaches the centre of the field.

Again, when the illumination is sufficiently faint, the whole of the retina, with the exception of the yellow spot, is totally colourblind. All the colours of the spectrum pass into grey when the light is made dim enough. When we pass from ordinary daylight into a dark room, we are not at first able to discern objects: but after a time the eye adapts itself to the faint illumination. It then becomes able to discern objects but not their colourtones. It sees everything in black and white. It has been experimentally ascertained that this twilight vision depends on the portions of the retina which surround the yellow spot. The yellow spot itself does not become adapted to the faint illumination. If a small patch of colour is seen only by means of the yellow spot, decreasing illumination causes the colour to disappear altogether, but does not transform it into a patch of grey. Cases have been carefully examined and recorded of persons who showed an entire want of sensibility to colourtones, not only under faint illumination, but under all conditions. They saw everything in black and white. In most of these pathological cases, though not in all, there is an alteration in the distribution of the intensity of light-sensation in the spectrum. For the normal eye the region of greatest brightness is that of yellow light; for the totally colourblind, it lies in the green rather than in the yellow portion of the spectrum. It is a notable fact that the spectrum, as seen under sufficiently faint illumination, shows the same change in the distribution of the brightness of its parts. The totally colourblind cannot for the most part bear illumination of ordinary strength. They can see well in a dim light, but are painfully dazzled by full light. This indicates that their ordinary condition is analogous to that of a normal person whose eyes have been adapted to twilight vision. Colour-Blindness is common to both cases. Probably a special visual apparatus is brought into play in twilight vision, and this is the only apparatus which in most cases exists in the eyes of the totally colourblind. Recent research seems to show that this special apparatus is constituted by the rods of the retina as distinguished from the cones.