124 SIE J. LtrBBOCK ON THE SENSE OF COLOB 



May 18. — In order to test the limits of vision at tlie red end of 

 the spectrum, I used the same arrangement as before, placing the 

 trough so that the extreme division was in the ultra-red, and the 

 second in the red. I then placed 60 Daphnias in the ultra-red. 

 After five minutes' exposure I counted them. There were in the 



Eed. Ultra-red. 



Exp. 1 54 5 



„ 2 56 4 



I now gave them four divisions — dark, red, ultra-red, and dark 

 gain. The numbers were : — 



Dai-k. 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 : — 



Eed. Ultra- red. 



Exp. 1 46 4 



„ 2 47 3 



„ 3 44 6 



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 with a really 

 dark space : — 



Dark. Eed. Ultra-red. 



Exp. 1 4 43 3 



„ 2 3 45 2 



Certainly, therefore, their limits of vision at the red end of the 

 spectrum seen approximately to coincide with ours. 



I then proceeded to examine their behaviour with reference to 

 the other end of the spectrum. 



I then shut them off from all the rays except the blue, violet, 

 and ultra-violet. The result was as follows ; — 



Exp. 



