218 PERCEPTION OF ULTRA-VIOLET RAYS. 



To our eyes both, as I say, are equally dark, and so they 

 would be to the Daphnias if their limits of vision were 

 the same as ours. As a matter of fact, however, the 

 Daphnias all collected in the part of the trough under 

 the water, and avoided that under the bisulphide of car- 

 bon, showing that this, therefore, was to them darker 

 than the other. I varied the experiments in several 

 ways, but always with similar results. Bichromate of 

 potash is also impervious to the ultra-violet rays, and 

 had the same effect. 



Not satisfied with this, I tried to test it in another 

 way. 



I took a cell, in which I placed a layer of five-per- 

 cent, solution of chromate of potash less than an eighth 

 of an inch in depth, and which, though almost colourless 

 to our eyes, completely cut off the ultra-violet rays. I 

 then turned my trough at right angles, so that I could 

 cover one side of the ultra-violet portion of the spectrum 

 with the chromate and leave the other exposed. The 

 numbers were as follows : — 



Side of the ultra- 

 violet covered with Side 

 chromate of potash, uncovered. Dark. 



Exp. 1 5 ... 55 ... 



I now covered up the other side. 



Exp. 2 3 ... 57 ... 



Again covered up the same side as at first. 



Exp. 3 4 ... 56 ... 



Again covered up the other side. 



Exp. 4 3 ... 57 ... 



May 19. — I again tried the same arrangement, re- 

 ducing the chromate of potash to a mere film, which. 



