BUT VERY SENSITIVE TO ULTRA-VIOLET RAYS. 207 
rays; but, on the other hand, that they are very sen- 
sitive to the ultra-violet rays, which our eyes cannot 
perceive. 
I then arranged the same ants in a wooden frame 
consisting of a base and two side walls, between which 
in the middle was a perpendicular sliding door. The 
pup had been arranged by the ants in the centre of 
the nest, so that some were on each side of the door. 
We then threw, by means of a strong induction-coil, a 
magnesium-spark on the nest from one side, and the 
light from a sodium-flame in a Bunsen burner on the 
other, the light being in each case stopped by the sliding 
door, which was pressed close down on the nest. In this 
way the first half was illuminated by the one light, the 
second by the other, the apparatus being so arranged 
that the lights were equal to our eyes—tkat, however, 
given by the magnesium, consisting mainly of blue, 
violet, and ultra-violet rays, that of the sodium being 
very yellow and poor inchemical rays. In a quarter of 
an hour the pup were all carried into the yellow. 
The sodium light being the hotter of the two, to 
eliminate the action of heat I introduced a water-cell 
between the ants and the sodium-flame, and made the 
two sides as nearly as possible equally light to my eye. 
The pup, however, were again carried into the sodium 
side. 
I repeated the same experiment as before, getting 
the magnesium-spark and the sodium-flame to the same 
degree of intensity, as nearly as my eye could judge, 
