200 THE ULTRA-RED AND ULTRA-VIOLET RAYS. 
the earth, they were not thoroughly satisfactory. Mr 
Spottiswoode was also good enough to enable me to 
make some experiments with electric light, which were 
not very conclusive; more recently I have made some 
additional and much more complete experiments, 
through the kindness of Prof. Dewar, Prof. Tyndall, 
and the Board of Managers of the Royal Institution, 
to whom I beg to offer my cordial thanks. 
Of course, the space occupied by the visible spec 
trum is well marked off by the different. colours. 
Beyond the visible spectrum, however, we have no 
such convenient landmarks, and it is not enough to 
describe it by inches, because so much depends on the 
prisms used. If, however, paper steeped in thalline is 
placed in the ultra-violet portion of the spectrum, it 
gives, with rays of a certain wave-length, a distinctly 
visible green colour, which therefore constitutes a green 
band, and gives us a definite, though rough, standard 
of measurement. 
In the above experiments with coloured spectra, 
the ants carried the pupz out of the portion of the 
nest on which coloured light was thrown and deposited 
them against the wall of the nest; or, if I arranged a 
nest of Formica fusca so that it was entirely in the 
light, they carried them to one side or into one corner. 
It seemed to me, therefore, that it would be interesting 
so to arrange matters, that on quitting the spectrum, after 
passing through a dark space, the ants should encounter 
not a solid obstacle, but a barrier of light. With this 
