76 SPECIAL SENSES. 



Red and greenish-blue. 

 Orange and cyanogen-blue. 

 Yellow and indigo-blue. 

 Greenish-yellow and violet. 1 



The fact that impressions made upon the retina persist for 

 an appreciable length of time enables us to illustrate the law 

 of complementary colors. If a disk, presenting divisions 

 with two complementary colors, be made to revolve so rapidly 

 that the impressions made by the two colors are blended, the 

 resulting color is white. 



It is almost useless, with our present knowledge, to specu- 

 late with regard to the probable mechanism of the apprecia- 

 tion of colors in vision. The facts just stated are sufficiently 

 clear, showing that the number of ethereal vibrations is dif- 

 ferent for different colors ; but it is by no means determined 

 that differences in the amplitude of the vibrations are in 

 direct relation with the arrangement of the disks of the rods 

 and cones in different portions of the retina. 2 The curious 

 phenomena of color-blindness depend upon an abnormal con- 

 dition of the visual apparatus. Persons possessing this pe- 

 culiarity called sometimes Daltonism, after the celebrated 

 English chemist, who described this infirmity as it existed in 

 his own person though vision may be normal in other re- 



1 TYNDALL, Light and Electricity, New York, 1871, p. 70. 



2 ZENKER, Versuch einer Theorie der Farben-Perception. Archiv fur mi- 

 kroskopische Anatomie, Bonn, 1867, Bd. iii., S. 248, et seq. In this article, Zen- 

 ker proposes the theory of the perception of colors referred to above ; but, as 

 far as we know, this has not been unreservedly adopted by any writers upon 

 physiological optics, though it must be admitted to be the only theory of color- 

 perception that approaches a satisfactory explanation of this most difficult ques- 

 tion. We are by no means prepared to deny in toto the proposition that the 

 perception of color is a question of situation, different colors being appreciated 

 by different portions of the retina ; and some of the facts with relation to color- 

 blindness are favorable to this view ; still, to render this certain, it is necessary 

 to establish a very exact relation between the length of the waves of light of 

 different colors and the diameter and number of the disks of the sensitive ele- 

 ments of the retina. For a very fair and full discussion of this theory, the 

 reader is referred to the admirable compendium of Kaiser. ( Compendium der 

 physiologischen Optik, Wiesbaden, 1872, S. 151, et seq.) 



