PROPERTIES OF THE RETINA. 325 



yellow, and the other shadow, that from the candle-light, will by 

 contrast seem quite blue. A striking instance of the effect of con- 

 trast is given, also, by the simple experiment of Mayer, illustrated 

 in Fig. 139. The gray square on the green background suffers no 

 apparent change from contrast, but if the figure is covered by a 

 sheet of white tissue paper the gray square at once takes on a red- 

 dish hue. It is evident that in all artistic and ornamental employ- 

 ment of colors this influence must be considered, and empirical 

 rules are established which indicate for the normal eye the bene- 

 ficial or the killing effect of different colors when brought into 

 juxtaposition. 



Color Blindness. The fact that some eyes do not possess 

 normal color vision does not seem to have attracted the attention 

 of scientific observers until it was studied with some care by Dalton, 

 the distinguished English chemist, at the end of the eighteenth 

 century. Dalton himself suffered from color blindness, and the 



Fig. 140 A. Black and white disc for ex- Fig. 140B. Showing the result when the 

 periment on contrast. (Rood.) disc A is set into rapid rotation. (Rood.) 



particular variety exhibited by him was for some time described 

 as Daltonism, but is now usually designated as red blindness. The 

 subject was given practical importance by later observers, espe- 

 cially by the Swedish physiologist Holmgren,* who emphasized its 

 relations to possible accidents by rail or at sea in connection with 

 colored signals. It is now the practice in all civilized countries to 

 require tests for color blindness in the case of those who in railways 

 or upon vessels may be responsible for the interpretation of signals. 

 The numerous statistics that have been gathered show that the 

 defect is fairly prevalent, especially among men. It is said that 

 on the average from 2 to 4 per cent, are color blind among males, 

 while among women the proportion is much smaller, 0.01 to 1 per 

 cent. Among the poorly educated classes the defect is said to be 



* Holmgren, "Color Blindness in its Relations to Accidents by Rail and 

 Sea," " Smithsonian Institution Reports," Washington, 1878. See also Jeffries, 

 "Color Blindness, its Dangers and its Detection," Boston. 



