OBJECTIONS OF M. MEKEJKOWSKY. 219 



however, still cut off the ultra-violet rays. I then placed 

 it, as before, over one half of the ultra-violet portion of 

 the spectrum ; and over the other half I placed a similar 

 cell containing water. Between each experiment I 

 reversed the position of the two cells. The numbers 

 wee — 



Evidently, then, even a film of chromate of potash 

 exercises a very considerable influence ; and, indeed, I 

 doubt not that, if a longer time had been allowed, the 

 difference would have been even greater. 



It seems clear, therefore, that a five-per cent, solution 

 of chromate of potash only one-eighth of an inch in 

 thickness, which cuts off the ultra-violet rays, though 

 absolutely transparent to our eyes, is by no means so to 

 the Daphnias. 



These observations seem to prove, though I differ 

 with great reluctance from so eminent an authority as 

 M. Paul Bert, that the limits of vision of Daphnias do 

 not, at the violet end of the spectrum, coincide with 

 ours, but that the Daphnia, like the ant, is affected by 

 the ultra-violet rays. 



Since these observations were published, M. Merej- 

 kowsky has experimented on the subject, and come 

 to the conclusion that the Daphnias are attracted 

 wherever there is most light, that they are conscious 

 only of the intensity of the light, and that they have no 

 power of distinguishing colors. It is no doubt true 

 that in ordinary diffused daylight the Daphnias generally 



