ance becomes extreme when one can not distinguish, for example, red from green, a cherry or a ripe strawberry amid the foliage, or a green from a red light in railway or ship signals. Artists sometimes have marked predilections for certain colors. Lesueur put a profusion of blue into all his paintings, and Turner seems to have sought and found red everywhere. It might be worth while to investigate whether the choice of their favorite colors by some painters is wholly intentional, or is a consequence of a physiological state. Color-blind persons are generally so by birth, but the affection may sometimes be the result of an accident. In some nervous affections it is occasionally manifested temporarily and under the strangest forms. The sight may thus, more than the other senses, be the victim of numerous errors and illusions. To speak only of those which have relation to colors, I notice the effects of contrast of two neighboring colors, or those which follow the impression of an image, or the subjective colors we see with our eyes shut, the result of a mechanical action on the eye, and I shall limit myself to describing some experiments relative to the apparent relief of colors. When we examine on a screen the image of a spectrum produced by a direct-vision prism, the successive colors appear as if situated on the same plane; but, if we slowly turn the slit or the prism, we shall have the illusion of a colored blade in relief with the red extremity forward. The effect is more sensible when the slit is V-shaped, in which case the spectrum resembles a groove. If we substitute for the slit the word DAVY in transparent letters, there will appear to be produced on the screen an exaggerated form of letters in relief like those we see on some shop-signs.
Outside of the colors we are accustomed to see, the solar spectrum includes other rays, some less refrangible than the red, which make themselves manifest by their calorific properties, and others more refrangible than the violet, which are remarked by their photographic effects, and by the action they exercise on fluorescent substances. The solar ultra-violet spectrum produced in the prism occupies an extent nearly equal to that of the luminous spectrum; while Mr. Stokes has shown that the electric arc gives an ultra-violet spectrum five or six times more extended.
We may be surprised that the sight of man is restricted to so small a part of the rays emitted by a luminous source. We have to remark on this point that the case is the same with the other senses. The touch can give an idea of the temperature of bodies only within very narrow limits; the ear can perceive neither extremely grave nor extremely acute tones, and the highest sounds it can hear produce a painful impression. On the side of the infra-red, the visible spectrum stops very abruptly, and the efforts of Brewster extended the range of the rays that the eye can perceive only slightly. Visibility, on the other side of the spectrum, persists in a remarkable manner. Helmholtz had already discovered that with certain precautions he
