AND SODIUM FLAME, 209 
It will be remembered that if an opaque substance is 
placed over a part of a glass nest, other things being 
equal, the ants always congregate under it; and that if 
substances of different opacity are placed on different 
parts of a nest, they collect under that which seems to 
them most opaque. Over one of my nests of Formica 
fusca, therefore, I placed two pieces of dark-violet glass’ 
4 inches by 2 inches; and over one of them I placed a 
cell containing a layer of bisulphide of carbon, an inch 
thick, slightly coloured with iodine. In all these experi- 
ments, when I moved the liquids or glasses, I gave 
the advantage, if any, to the one under which experi- 
ence showed that the ants were least likely to congre- 
gate. The ants all collected under the glass over 
which was the bisulphide of carton. 
I then thought that though no doubt the iodine 
rendered the bisulphide more completely impervious to 
the ultra-violet rays, I would try the effect of it when 
pare and perfectly colourless. I therefore tried the 
same experiment with pure bisulphide, moving the two 
glasses from time to time in such a manner that the 
ants had to pass the first violet glass in order to reach 
that over which was the bisulphide. 
At 8.30 the ants were all under the glass over which 
was the bisulphide of carbon: I then changed 
the position. 
8.45 ” 9 23 
9 ” ” ” 
9.15 9 ” ” 
