856 PERCEPTION OF COLORS. 



PERCEPTION OF COLORS. 



Physical Considerations. The undulations of the hmriniferous ether are 

 perceived by the retina only within definite limits. If a beam of white light, 

 for example from the sun, be allowed to pass through a prism, its rays are refracted, 

 and are decomposed into the prismatic spectrum (Fig. 12) . The white light contains 

 rays of widely different wave-length, or number of vibrations. The dull heat- 

 rays are the least refracted; their wave-length measures 0.00194 mm.; they do 

 not affect the retina, and are, therefore, invisible, although as is well known they 

 affect the sensory nerves. About 90 per cent, of these rays are absorbed by the 

 ocular media. Commencing at Fraunhofer's line A (Fig. 15) the oscillations 

 of the ether excite the retina, and the colors appear in the following order: red, 

 with 481 billions of vibrations in a second; orange, with 532; yellow, with 563; 

 green, with 607 ; blue, with 653 ; indigo, with 676 ; and violet, with 764 billions in a 

 second. The perception of color depends, therefore, upon the number of vibra- 

 tions of the ether, just as the pitch of a note depends upon the number of vibra- 

 tions of the sounding body. The heat-rays that lie in the colored spectrum are 

 transmitted by the ocular media in about the same way as by water. Beyond 

 the violet rays lie the chemically active or actinic rays. The ultra-violet rays are 

 largely absorbed by the ocular media, especially by the lens. On shutting off 

 the entire spectrum, including the violet rays, the ultra-violet rays may yet be 

 recognized from their pale, grayish-blue color. The ultra-violet rays can be most 

 easily demonstrated by the phenomenon of fluorescence: on illuminating a solution 

 of quinin sulphate with ultra-violet v. Helmholtz saw a bluish-white light arise 

 from all parts of the solution that were reached by ultra-violet rays. As the 

 ocular media themselves exhibit fluorescence, they must increase the power of 

 the retina to distinguish these rays. 



In order that color may be recognized, it is necessary for a certain 

 amount of light to fall upon the retina. The lowest degree of brightness 

 by which blue may be recognized as a color is 1 6 times less than that 

 required by red. If, therefore, in a bright illumination, a red and a blue 

 object appear equally bright, the blue will appear brighter as soon as the 

 illumination is decreased: Purkinje's phenomenon. The retina is 

 least readily stimulated by red, and the variations in intensity of red 

 are recognized with the greatest difficulty. Therefore, according to 

 Briicke, intermittent white light is perceived as greenish, because the red 

 component in white light acts upon the retina with greater difficulty. 

 Yellow, on the contrary, acts more powerfully, and then follows blue. 

 In weak illumination green possesses the greatest brightness ; then come 

 yellow, blue, red. In strong illumination the analogous succession of 

 colors is: yellow, red, green, blue. 



While, therefore, light of varying rapidity of vibration produces in the eye the 

 sensation of different colors, the amplitude of vibration (height of the waves) deter- 

 mines the intensity of the visual impression, just as the loudness of a note depends 

 upon the amplitude of the vibrations of the sounding body. All of the colors are 

 united in sunlight, and their simultaneous action on the retina produces the 

 sensation that is designated as the sensation of white. If the colors of the 

 spectrum obtained by means of a prism are again united, white light is once 

 more produced. If the retina is not influenced by vibrations of the luminiferous 

 ether, all sensation of light and of color is absent ; but this cannot be designated 

 black It is rather the absence of sensation, as is the case also when a ray of 

 light falls on the skin of the back. The skin perceives neither black nor any 

 light-sensation whatever. 



When a colored object is illuminated by a monochromatic light, it gives no 

 impression of color. If a colored object is illuminated by two lights of different 

 color, the color-impression appears best if one of the lights contains those rays 

 that would be most strongly reflected by the color of the object, and the other 

 light, on the contrary, contains such rays as stand closer to the color in the solar 

 spectrum than does the complementary color. 



