208 EXPERIMENTS WITH MAGNESIUM SPARK 
and interposing a water-screen between the sodium- 
flame and the ants. The temperature was tested by 
the thermometer, and I could distinguish no difference 
between the two sides. Still the ants preferred the 
sodium side. This I repeated twice. I then removed 
the magnesium-spark somewhat, so that the illumina- 
tion on that side was very much fainter than on the 
other; still the pups were carried into the sodium- 
light. I then turned the nest round so as to bring 
them back into the magnesium. They were again 
carried to the sodium side. 
Once more I repeated the same experiment. The 
light on the magnesium side was so faint that I could 
scarcely see the pupz, those on the sodium side being 
quite plain. The thermometer showed no difference 
between the two sides. The pupz were carried into 
the sodium-light. Ithcn turned the nest round twice ; 
but the pupz were each time carried out of the 
magnesium-light. 
These experiments seemed strongly to indicate, if 
not to prove, that ants were really sensitive to the 
ultra-violet rays. Now to these rays sulphate of 
quinine and bisulphide of carbon are extremely onaque, 
though perfectly transparent in the case of visible rays, 
and therefore to our eyes entirely colourless and trans- 
parent. If, therefore, the ants were really affected by 
the ultra-violet rays, then a cell containing a layer of 
sulphate of quinine or bisulphide of carbon would tend 
to darken the underlying space to their eyes, though 
to ours it would not do so. 
