686 THE POPULAR SCIENCE MONTHLY. 



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 appar- 

 ent 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 re- 

 mark 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 spec- 

 trum 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 



