200 THE ULTRA-EED AND ULTRA-VIOLET RAYS. 



the earth, they were not thoroughly satisfactory. Mr. 

 Spottiswoode was also good enough to euable 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 pupae out of the portion of the 

 nest on which coloured light was thrown and deposited 

 them agaiiist 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 comer. 

 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 thia 



