FLUORESCENCE. 75 



and showing that the peculiar coloured light exhibited 

 in this and other allied instances, which were matters of 

 ordinary observation, 1 had an origin hitherto unsuspected 



I have here some phosphorescent tubes which have 

 been lent to me, which you will see retain the luminosity 

 for some time in the dark after the light of the electric 

 lamp has been allowed to play upon them. The most 

 phosphorescent substances are certain sulphides of metal-s 

 of alkaline earths, though it is said that certain impurities 

 contribute to the effect rather than otherwise. 



In the course of my experiments I was led to see that 

 glass was by no means transparent in regard to the most 

 refrangible of the rays I had to deal with. I procured 

 accordingly prisms and lenses of quartz, with which to form 

 a pure spectrum. On applying them to the solar spectrum, 

 the invisible rays were seen to extend far beyond anything 

 I had ever seen before, and showed a continuation of 

 Fraunhofer's lines that is represented on these maps, 

 which were originally drawn for an evening lecture I 

 gave on the subject for the British Association at 

 Belfast in 1852. In the spring of the next year, while 

 preparing for a lecture at the Royal Institution, in the 

 laboratory of that institution, along with Faraday, though 

 I had expected beforehand to obtain a very long spectrum 

 by the use of the electric light, I was utterly surprised 

 when I found the actual length of it. I used in the first 

 instance a Ley den jar, and I could not but think at the first 

 moment that there was some stray reflection of light, for if 

 the visible spectrum was about one inch, the whole spectrum 

 obtained by means of powerfully fluorescent substances, 

 such as uranium glass, with the electric light, was about 

 six or eight inches, which was a length I had not been at all 

 prepared for. 



1 The very name " Schillerstoff, " formerly given to sesculin, is 

 derived from the property in question. 



