352 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



most animals, other than man and the higher primates, the retina 

 has an extra coat or layer, the tapetum lucidium. This may be the 

 reflecting surface. 



The editor of the Journal of the Bombay Natural History Society 

 (vol. 31, No. 1, p. 221) makes a brief comment regarding a short 

 article by A. A. Dunbar Brander relative to the source of color in 

 the eye of the gaur, which reads: 



In volume IV of the Society's Journal the late Mr. J. D. Inverarity came to 

 the same conclusion as Mr. Dunbar Brander and pointed out that the blue 

 colouring of the gaur's eye is due to the tapetum lucidium — the lining to a greater 

 or less extent of the back part of the choroid membrane of the eye which, in the 

 gaur, is of a lovely peacock-blue color. It is this membrane which causes an 

 animal's eye to shine in the dark. In the human eye it is opaque and black. 



The eyes of most domesticated and some wild animals deteriorate 

 in captivity so that the vision may become very poor. Defective eyes 

 have been apparent in a few of the cases observed. 



At the suggestion of Dr. Earl S. Johnston, of the Smithsonian Insti- 

 tution, Division of Radiation and Organisms, beams of red and blue 

 light have been projected into the eyes. These rays were obtained 

 by fastening a single thickness of red or blue Cellophane over the 

 flashlight lens. Over 40 species of mammals, reptiles, and amphibians 

 were examined under the red rays, but there was very little variation 

 in colors of the reflected light from various animals other than the 

 addition of a red tinge. They varied from a dull reddish amber 

 through reddish silver to reddish gold, the differences being mainly 

 those of brilliance. Observations on 30 species of mammals and rep- 

 tiles gave similar results with blue rays, the reflections ranging from 

 dull and pale bluish silver to blue-green and blue, with the metallic 

 luster persisting in those species that give brilliant reflections under 

 normal light. The reflections from the crocodilians were partially 

 opalescent. The characteristics of the responses to colored lights indi- 

 cate that fluorescence does not account for the response to the rays of 

 the torch. Reflection is left as the only probable source of the return 

 light. 



In some animals the color of the "shine" is constant whereas in 

 others it may appear as three different colors in a few seconds, while in 

 some instances the color is constant for each observation but may be 

 different on different days. 



It has been suggested that the change is caused by the animal's 

 changing the direction of its eyes. In some cases this is true, but in 

 others I have been unable to detect any change in the position of the 

 eye, and since most animals change their line of sight mainly by moving 

 the head rather than rolling the eyes it is probable that a change that 

 would alter the reflected light would be detected in movement of the 

 head. Most of the bears that I have observed swing their heads from 

 side to side without changing the color of the reflections from their eyes. 



