216 PERCEPTION OF ULTRA-VIOLET RAYS. 



except the blue, violet, and ultra-violet. The result 

 was as follows : — 



Number op Daphnias. 



Ultra-violet. Violet. Blue. Dark. 



Exp. 1 1 9 38 2 



„ 2 4 6 38 2 



„ 3 2 46 2 



5 17 122 6 



This shows that they greatly prefer blue and violet to 

 darkness or ultra-violet. 



I afterwards gave them only the option of ultra-violet, 

 violet, and darkness : — 



Exp. 1 



„ 2 



« s 



„ 4 



., 5 



45 238 17 



They preferred the violet; but there were many 

 more in the ultra-violet than in the dark. 



I then tried ultra-violet and dark. The width of the 

 violet was two inches; and 1 divided the ultra-violet 

 portion again into divisions each of two inches, which 

 we may call ultra-violet, further ultra-violet, and still 

 further ultra-violet. The results were — 



Number op Daphnias. 



Exp. 



286 14 



