LIMITS OF VISIOX OF DAPHNIAS. 215 



red end of the spectrum, I used the same arrangement 

 as before, phiciiig the trough so that the extreme 

 division was in the ultra-red, and the second in the red. 

 I then placed sixty Daphnias in the ultra-red. After 

 five minutes' exposure, I counted them. There were in 

 the— 



Red. Ultra- red, 



Exp. 1 54 ... 5 



„ 2 56 ... 4 



I now gave them four divisions to select from — dark, 

 red, ultra-red, and dark again. The numbers were — 



Dark. Red. Ultra-red Dark. 



Exp. 1 5 47 6 2 



„ 2 9 41 7 3 



I then shut them off from all the colors excepting 

 red, giving them only the option between red and 

 ultra-red : — 



Exp. 1 



« 2 



„ 3 



I then left them access to a division on the other side 

 of the red, which, however, I darkened by interposing a 

 piece of wood. This enabled me better to compare the 

 ultra-red rays wath a really dark space : — 



Exp. 1 



„ 2 



These observations appear to indicate that their 

 limits of vision at the red end of the spectrum coincide 

 approximately with ours. 



I then proceeded to examine their behaviour with 

 reference to the other end of the spectrum. 



In the first place, I shut them off from all the rays 



