THE ULTRA-VIOLET RAYS. 203 



if a ray of white light is passed through a prism, it is 

 broken up into a beautiful band of colors, known as the 

 spectrum. To our eyes this spectrum, like the rainbow, 

 which is, in fact, a spectrum, is bounded by red at the 

 one end and violet at the other, the edge being sharply 

 marked at the red end, but less abruptly at the violet 

 But a ray of light contains, besides the rays visible to 

 our eyes, others which are called, though not with 

 absolute correctness, heat-rays and chemical rays. 

 These, so far from falling within the limits of our vision, 

 extend far beyond it, the heat-rays at the red end, the 

 chemical or ultra-violet rays at the violet end. 



I made a number of experiments which satisfied me 

 that ants are sensitive to the ultra-violet rays, which 

 lie beyond the range of our vision. I was also anxious 

 to see how two colors identical to our eyes, but one 

 of which transmitted and the other intercepted the 

 ultra-violet rays, would affect the ants. 



Mr. Wigner was good enough to prepare for me a 

 solution of iodine in bisulphide of carbon, and a second 

 of indigo, carmine, and roseine mixed so as to produce 

 the same tint. To our eyes the two were identical both 

 in color and capacity; but of course the ultra-violet 

 rays were cut off by the bisulphide-of-carbon solution, 

 while they were, at least for the most part, transmitted 

 by the other. I placed equal amounts in flat-sided 

 glass bottles, so as to have the same depth of each 

 liquid. I then laid them, as in previous experiments, 

 over a nest of Formica fusca. In twenty observations 

 the *ants went seventeen times in all under the iodine 

 and bisulphide, twice under the solution of indigo 

 and carmine, while once there were some under each. 

 These observations, therefore, show that the solutions. 



