74 LECTURES TO SCIENCE TEACHERS. 



which has been treated in this manner. I may mention 

 also that even a simple revolving mirror will show the 

 duration of the effect in such cases as the salts of uranium 

 and solids in general. If you use an ordinary electric 

 machine as a source of illumination, giving a succession of 

 sparks, or an induction coil with a Leyden jar in connection 

 with the two terminals, which gives a momentary discharge, 

 you can observe the substance in a rapidly revolving mirror, 

 and in that way you get a momentary view of the substance 

 by reflected light, while the illumination due to fluor- 

 escence, in case it has an appreciable duration, is drawn out 

 into a broad gleam ; so that even without an instrument of 

 the kind now on the table the duration of the effect can 

 be manifested by experiment. The duration of the elfect, 

 I may observe, has not as yet in any instance been 

 demonstrated experimentally in the case of a liquid. 



When epipolic dispersion wa,s referred to a change of 

 refrangibility of light, there were some older experiments 

 which at once received their explanation. For instance, 

 Sir John Herschel himself, on throwing a pure spectrum 

 on turmeric paper, had noticed a great prolongation of the 

 ordinary visible spectrum. But he supposed that this was 

 due to the ultra violet rays, which were directly reflected 

 into the eye ; and he speculated whether there might not be 

 a repetition of the colours of the ordinary spectrum. This, 

 however, proved to be a phenomenon of the kind I have 

 just described, fluorescence, or, if you like so to call it, 

 phosphorescence, as I am persuaded that fluorescence is 

 nothing but phosphorescence of brief duration. Mons. E. 

 Becquerel went still nearer to the actual phenomenon, 

 for he was making experiments on substances which show 

 phosphorescence after exposure to light, and observed that 

 some of them were specially luminous when light fell upon 

 them and was acting upon them. Nevertheless, although he 

 correctly explained what he witnessed in these cases, from 

 connecting it too closely with phosphorescence, he failed to 

 perceive the full bearing of his own observation ; and 

 though he had actually under his hands the solution of 

 quinine, and had discovered by means of photography 

 the intense absorbing action of that fluid on the invisible 

 rays, and expressly mentioned the " dichroism " of the solu- 

 tion he never dreamt of putting the two things together, 



