The theory which supposes red, green, and violet to be Man's three primary colours is the hypothesis of our great countryman Dr. Thomas Young, and deserves much more consideration than was for a long time awarded it. If we may judge of his theory by his appreciation of pictures it must have been excellent;—the present writer saw with admiration in 1845 the grand series of Reynolds' portraits which Dr. Young had left behind him.
The second topic of interest is the inquiry into the number and tone of subjective colours. A perfect theory of colour ought, of course, to embrace all possible human sensations of the kind. Now many persons are able to see in dreams a rich amber light far softer and more pure than any tint ever beheld by the Eye. It generally appears to irradiate Space, and silvery figures, most often the celestial orbs, float within it. A still more beautiful production of reflex energy exerted after tranquil rest is the blending of delicate green with a hyacinthine hue quite strange to this world, and indescribably lovely in its tender shadings off. By means of this subjective activity the experiments of Goethe and J. Müller may be varied almost ad libitum. The easiest plan is on first waking to keep the eyelids steadily closed, and watch for the unbidden rise of tints. Persons of strong pictorial and poetic powers can, after some practice, control their appearance and succession; and much diversity may be produced by slightly separating the fringe of eyelashes and looking between the loosely pressed fingers. The remarkable point in these and similar experiments seems to be that we are thus enabled to gaze upon beauties more marvellous than the outward eye ever beheld—yet we see them.
Another and a painful source of knowledge on this subject consists in registering the visual impressions of persons bodily or mentally diseased. The difference between these and the normal impressions of healthy people would seem to arise from reflex action, the disordered sensory or mind reacting upon the optic apparatus; or, as it may be said, the centre of our being is through these aberrations made manifest in its control of the circumference.
Now, it will be obvious to any reflective person how very important all information we can acquire respecting this central empire over the impressions of our sense-nerves may become when we try to estimate the conditions of human knowledge. If it be true that the Mind imposes laws of activity on the nervous system even when receiving impressions from it, then the necessity we are under of thinking in accordance with certain inly imposed laws receives a most striking illustration. And the inference from it carries an a fortiori probability since our thoughts lie nearer to our mental centre than any of our sense-impressions.
[99] Nerves of common feeling are acutely sensitive when divided, and the patient animal under a Majendie or a dentist utters a sharp shriek. The case is different with motor nerves, with those of the sympathetic system, and with (what is more to our purpose) nerves of sensation. It seems clear that mechanical injuries, or even touches, excite them in the direction of their own special functions. Auditory nerves feel a shock as a sound,—optic nerves receive it as a sudden and brilliant light. We are doubly assured from these effects of the true functions belonging to the several sets of nerves. Disease and injury are great discoverers of what ought to be healthy susceptibilities. In such cases, however, they prove also something more agreeable to think upon. They prove that suffering is confined within definite limits, and that economy of pain forms part of the universal design, for the sensitive animal as well as the sensitive man. If all our nerves shrank equally with equal tenderness, life would be a history of protracted agony. Yet one might have expected, primâ facie, that a fibre which telegraphs shapes and colours with their blendings, would eloquently tell the story of its own occasional anguish. And our whole nervous framework might have been conceived as an instrument of torture. It has not been so constituted.
Per contra, the nerves of common feeling assert their own vocation.—"A brazen canstick turned" sets the teeth on edge, and troubles the skin with horripilation. Believers in ghosts—and also disbelievers—are aware that some sights
"Make knotted and combined locks to part,
And each particular hair to stand on end."
For extended information on this subject compare Additional Note C.
[100] Aristotle so described it before Mr. Bain and other modern writers, "τὸ γὰρ ὁρατόν ἐστι χρῶμα," De Anima II. 7. 1. As Kampe carefully observes, "so ist die Farbe (nicht die gefärbten Körper) das Eigenthümliche des Gesichtssinns." See also his note, Erkenntnisstheorie des Aristoteles, p. 88.