Animal Life and Intelligence

by C. Lloyd Morgan

"1. Red excites strongly the fibres sensitive to red, and feebly the other two.

"2. Yellow excites moderately the fibres sensitive to red and green, feebly the violet.

"3. Green excites strongly the fibres sensitive to green, feebly the other two.

"4. Blue excites moderately the fibres sensitive to green and violet, feebly the red.

"5. Violet excites strongly the fibres sensitive to violet, feebly the other two.

"6. When the excitation is nearly equal for the three kinds of fibres, the sensation is white."

This theory cannot be regarded as more than a provisional hypothesis. Still, by its means we can explain many colour-phenomena. It is well known, for example, that if we gaze steadily at a red object, and then look aside at a grey surface, an after-image of the object will be seen of a blue colour. According to the theory, the red fibres have been tired and cannot so readily answer to stimulation. Over this part of the retina, therefore, the effect of grey light is to stimulate normally the fibres sensitive to green and violet, but only slightly those sensitive to red, owing to their tired condition. The result will be, as we see from the above scheme (4), the sensation of blue. Colour-blind people, on this view, are those in whom one set of the fibres, generally the red or the green, are lacking or ill developed.

We may, perhaps, with advantage restate this theory in terms of chemical change, or metabolism. On this view three kinds of "explosives" are developed in the retinal cones; for it is seemingly the cones, rather than the rods, which are concerned in colour-vision. All three explosive substances are unstable; but one, which we may call R., is especially unstable for the longer waves of the spectrum; another, G., for the waves of mid-period; a third, V., for those of smallest wave-length.

Suppose that R. only were developed. If, then, we were to look at a band of light spread out in spectrum wave-lengths, we should see a band[FE] of monochromatic r. light. Its centre would be bright, and here would be the maximum instability of R. On either side it would fade away. The lateral edges of the spectrum would be the limits of the instability of R. If G. only were developed, we should see only a band of monochromatic g. light. Its centre would not coincide with that for R., but would lie in a region of smaller wave-length. Here would be the maximum instability for G. On either side the green would fade away. Its lateral edges would mark the limits of the instability of G. But though their centres would not coincide, the R. band and the G. band would to a large extent overlap. Similarly with the band for V. It, too, would have its centre of maximum instability and its lateral edges of lessening instability. Its centre would lie in a region of yet smaller wave-length than that for G. And the v. band would overlap the green and the red.

Normally, all three bands are developed, and their blended overlapping gives the colours of the rainbow. For this reason the monochromatic bands r., g., and v. are unknown to us in experience. All the colour-tints we know are blended tints. What we call full-red light causes strong disruptive change in R., but decomposes slightly G., and probably also, but in much less degree, V.