BEHAVIOR OF VERTEBRATES 433 



time was allowed for adaptation, and the animal was again 

 tested and the diaphragm again adjusted until the animal 

 would snap at the food. Several determinations of this kind 

 enabled the author to trace the whole course of adaptation to 

 darkness. 



The range of adaptive increase in sensitivity in the turtle is 

 considerable. For example, it is so great that a dark-adapted 

 animal can see a piece of moving meat under such conditions 

 of illumination that it is invisible to the bright-adapted human 

 eve. Hess finds that the whole course of adaptation is almost 

 identical with that of the human eye when the latter is covered 

 by a suitable orange-colored glass. The reviewer was not able 

 to find in the article a careful description of the orange-colored 

 glass used. 



Hess made tests upon the limits of the spectrum and the 

 brightness distribution in the spectrum, and traced the course 

 of adaptation to colored light by a new method. An arc spec- 

 trum 20-40 cm. long, and 5-10 cm. high, was projected hori- 

 zontally upon a dead-black surface. The turtles were placed 

 upon a table with their backs to the window in the dark-room 

 which admitted the spectrum. Food (white fish-meat, cooked 

 rice, and in most cases, wads of cotton wool), suspended from 

 a dead black wire, was dangled in the light in front of the animals. 

 The food could easily be changed from one region of the spectrum 

 to another. By moving the food over into the infra-red it was 

 quickly determined that the width of the animal's spectrum 

 at the red end was almost exactly the same as that of the normal 

 human eye. Further tests on the violet end show that the 

 reptiles have a very much shorter range there even than the 

 hen. His final conclusion is that both hen and turtles see colors 

 as we do, if we cover the eyes with a suitable red-yellow glass. 

 There is this difference between hen and turtle, however: The 

 human eye must be covered with a bright red-yellow glass 

 which admits some of the green and blue-green wave-lengths, 

 if it is to see the world of colors seen by the hen. It must be 

 covered with a dark glass lying somewhat further along in the 

 red region than the one just described for the hen, and it must 

 restrain the short wave-lengths — i. e., admitting those from the 

 yellow only, if it is to view colors as seen by the turtle. I give 

 these general statements because all of Hess's experimental re- 



