Let a person place two threads respectively red and green near the bridge of the nose, so as to be seen by the inner angle of the pupil only. If dexterously moved, both seem green;--if not, both will in time become black. Where the want of sensitive appreciation of red is great, the same result follows in every part of the field of sight. Thus reverend gentlemen in former times have been induced to wear scarlet hose under the impression that they had put on black silk; and in these railroad days many persons find themselves unable to distinguish between the safety and the danger signal lights. It seems strange indeed that any scientific advisers of railway Boards should have recommended for use the two colours, above all others, most likely to get confounded. 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.