The following table presents these observations and a few others in another light:—
| Violet | Red | No. of Individuals. | |||||
| Highest | Mean | Lowest | Highest | Mean | Lowest | ||
| Women under 30 | 410.0 | 402.75 | 394.0 | 811 | 772.85 | 743 | 17 |
| Men"" | 413.0 | 405.0 | 399.0 | 811 | 772.8 | 743 | 17 |
| Women over 30 | 410.5 | 406.65 | 401.5 | 792 | 767.8 | 743 | 7 |
| Men"" | 407.0 | 404.5 | 402.5 | 787 | 773.7 | 758 | 3 |
N { right eye | 406 | 687 | |||||
| 407 | 717 | ||||||
The individual N showed signs of colour-blindness, and is therefore not included in the table, but entered separately. He was unable to recognize the C line of the hydrogen spectrum (wave-length 656), which was brilliantly obvious to the normal eye.
These observations[FG] need further confirmation and extension. We intend to continue the investigation each session. They are, however, sufficient to show that in some individuals R. undergoes disruptive change on the impact of light-waves which have no noticeable effect on the retina of other individuals.
It is impossible here to do more than just touch the fringe of the difficult subject of colour-vision. And the only further fact that can here be noticed is that trichromatic colour-vision is apparently in us limited to the yellow-spot and its immediate neighbourhood. Around this is an area which is said to be bichromatic—all of us being, for this area, more or less green-blind. In the peripheral area around this, colour is indistinguishable, and we are only sensitive to light and shade. So far as the structure of the retina is concerned, we may notice in this connection that in the central region of most complete trichromatic vision there are cones only; around the yellow spot each cone is surrounded by a circle of rods; and further out into the peripheral region by two, three, or more circles of rods.
Concerning the sense of sight in the lower mammals little need be said. In many cases the acuteness of vision is remarkable. Mr. Romanes's experiments on Sally, the bald-headed chimpanzee at Regent's Park, led him to conclude that she was colour-blind, but I question whether the experiments described quite justify this conclusion. Sir John Lubbock was unable to teach his intelligent dog Van to distinguish between coloured cards; but the failure was as complete when the cards were marked respectively with one, two, or three dark bands. We are not justified, therefore, in ascribing the failure to colour-blindness. The real failure, probably, was in each case to make the animal understand what was wanted. Bulls are, at any rate, credited with strong colour-antipathies, and insect-eating mammals are probably not defective in the colour-sense.
It is said that nocturnal animals, such as mice, bats, and hedgehogs, have no retinal cones; and if the cones are associated with colour-vision, they may not improbably be unable to distinguish colours. Some moles are blind (e.g. the Cape golden mole). But the common European mole, though the eyes are exceedingly minute (1/25 of an inch in diameter), has the organ fairly developed, and is even said not to be very short-sighted. It is protected by long hairs when the animal is burrowing, and is only used when it comes to the surface of the ground.
It is probably in birds that vision reaches its maximum of acuteness. A tame jackdaw will show signs of uneasiness when seemingly nothing is visible in the sky. Presently, far up, a mere speck in the blue, a hawk will come within the range of far-sighted human vision. Steadily watch the speck as the hawk soars past, until it ceases to be visible; the jackdaw will still keep casting his eye anxiously upward for some little time. He may be only watching for the possible reappearance of the hawk. But just as he saw it before man could see it, so probably he still watches it after, to man's sight, it has become invisible. So, too, for nearer minute objects, the swift, as it wheels through the summer air, presumably sees the minute insects which constitute its food. And every one must have noticed how domestic fowls will pick out from among the sand-grains almost infinitesimal crumbs.
It is probable that the area of acute vision is much more widely diffused over the retina of birds than it is with us. In any case, the cones are more uniformly and more abundantly distributed over the general retinal surface.
An exceedingly interesting and important peculiarity in the retina of birds, which they share with some reptiles and fishes, is the development, in the cones, of coloured globules. "The retinæ of many birds, especially of the finch, the pigeon, and the domestic fowl, have been carefully examined by Dr. Waelchli, who finds that near the centre green is the predominant colour of the cones, while among the green cones red and orange ones are somewhat sparingly interspersed, and are nearly always arranged alternately—a red cone between two orange ones, and vice versâ. In a surrounding portion, called by Dr. Waelchli the red zone, the red and orange cones are arranged in chains, and are larger and more numerous than near the yellow spot; the green ones are of smaller size, and fill up the interspaces. Near the periphery the cones are scattered, the three colours about equally numerous and of equal size, while a few colourless cones are also seen. Dr. Waelchli examined the optical properties of the coloured cones by means of the micro-spectroscope, and found, as the colours would lead us to suppose, that they transmitted only the corresponding portions of the spectrum; and it would almost seem, excepting for the few colourless cones at the peripheral part of the retina, that the birds examined must have been unable to see blue, the whole of which would be absorbed by their colour-globules."[FH]