1859.] on Phosphorescence, Fluorescence, 6fc. 161 



generally possessed a lower degree of refrangibility than the ray causing 

 the phosphorescence ; but in some instances he was able to raise the ' 

 refrangible character of the ray emitted to that of the exciting ray. 

 Viy taking a given preparation, and raising it to different temperatures, 

 he caused it to give out different coloured rays by the single action of 

 one common ray ; this variation in power returning to a common 

 degree as the temperatures of the phosphori became the same in all. 

 lie showed that time was occupied in the elevation of the phosphores- 

 cent state by the ray ; and also that time was concerned in various 

 degrees during the emission of the phosphorescent ray : that this time, 

 which in many cases was long, might be affected, being shortened by 

 the action of heat, and then the brilliancy of the phosphorescence for 

 the shortened time was increased. He showed the special relation of 

 the different phosphori to the different rays of the spectrum, pointing 

 out where the maximum effect occurred ; also that there were the 

 equivalents of dark bands, Le, bands in the spectrum, where little or 

 no phosphorescence was produced. 



These phosphori were many of them highly fluorescent. Thus, if 

 one of theni was exposed to the strong voltaic light, and then placed 

 in the dark, it was seen to be brilliantly luminous, gradually sinking in 

 brightness, and ultimately fading away altogether : but if it were held 

 in the rays beyond the violet end of the spectrum (the more luminous 

 rays being shut off) it was again seen to be beautifully luminous, but 

 that state disappeared the instant it was removed from the ray. Now 

 this is fluorescence, and the same body seemed to be both phosphorescent 

 and fluorescent. Considering this matter, and all the circumstances 

 regarding time, Becquerel was led to believe that these two luminous 

 conditions differed essentially only in the time during which the state 

 excited by the exposure to light continued ; that a body being really 

 phosphorescent, but whose state fell instantly, was fluorescent, giving 

 out its light while the exciting ray continued to fall on it, and during 

 that time only ; and thata phosphorescent was only a more sluggish, 

 body, which continued to shine after the exciting ray was withdrawn. 

 To investigate this point he invented the phosphoroscope ; an apparatus 

 which may vary in its particular construction, but in which discs or 

 other surfaces illuminated by the sun or an electric lamp, might, by 

 revolution, be rapidly placed before the eye in a dark chamber, and so 

 be regarded in the shortest possible space of time after their illumina- 

 tion. By such an apparatus Becquerel showed that all the fluorescent 

 bodies were really phosphorescent ; but that the emission of light 

 endured only for a very short time. 



An extensive series of experimental illustrations upon the foregoing 

 points was made with fine specimens of phosphori, for which the speaker 

 was indebted to M. Becquerel himself. The phosphoroscope employed 

 consisted of a cylinder of wood, one inch in diameter and seven inches 

 long, placed in the angle of a black box with the electric lamp inside, 

 so that three-fourths of the cylinder were external, and in the dark 

 chamber where the audience sat, and one-fourth was within the box, and 

 in the full power of the voltaic light. By proper mechanical arrange- 



