126 SIE J. LUBBOCK OK THE SENSE OE COLOE 



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



I placed over the ultra-violet division a glass cell contain- 

 ing a layer of sulphate of quinine about | inch in depth, and 

 over the further ultra-violet a similar cell with water. I had 

 expected that the great majority would collect under the water-cell. 

 The numbers, however, were : — 



Further ultra-Tiolet, Ultra-violet, with 

 with cell containing cell containing 



water. sulphate of quinine. 



Exp. 1 8 50 



„ 2 4 54 



„ 3 11 49 



„ 4 4 56 



The reason of this, however, seemed evident as soon as I tried 

 the experiment, because, though the sulphate of quinine stops 

 the ultra-violet rays, it turns them into blue light, and, to our 

 eyes at least, actually increases the brilliance. 



I then took a cell in which I placed a layer of 5-per-cent. solution 

 of chromate of potash less than an eighth of an inch in depth, which, 

 though almost colorless 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. 

 „ 2 3 57 



Again covered up the same side as at first. 

 „ 3 4 56 



Again covered up the other side. 

 „ 4 3 57 



May 19. — Again the same arrangement. I reduced the chro- 

 mate of potash to a mere film, which, 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 experi- 

 ment I reversed the position of the two cells. The numbers 

 were : — 



