PROPERTIES OF THE RETINA. 333 



where it borders on a lighter circle, and lighter on its inner edge where 

 it borders on a darker circle. Similar contrasts may be obtained from 

 comparing shadows cast by yellow and white light. If a rod be 

 arranged in a dark room so as to cast a shadow from an opening 

 admitting daylight and one also from a lighted candle, either shadow 

 taken separately appears black, but if the two are cast side by side 

 one will appear blue, the other yellow. The shadow cast by the 

 daylight, being illuminated by the j-ellow candle-light, will appear 

 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. 141. 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- 



Fig. 142^. Black and white disc for ex- Fig. 142B. Showing the result when the 

 penment on contrast. (Rood.) disc --i is set into rapid rotation. (Rood.) 



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 

 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 



* 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. 



