NATURAL PHILOSOPHY. 147 



which are stopped or consumed in the act of rendering the fluorescent body 

 luminous; so they produce this effect only at the first or entry surface, the 

 passing ray, though the light goes onward, being unable to produce the 

 effect again ; and this effect exists only whilst the competent ray is falling 

 on to the body, for it disappears the instant the fluorescent substance is 

 taken cut of the light, or the light shut off from it. When E. Bccquerel 

 attacked this subject, he enlarged it in every direction. First of all, he pre- 

 pared most powerful phosphori, these being chiefly sulphurets of the alka- 

 line earths, strontia, baryta, lime. By treatment and selection, he obtained 

 them so that they would emit a special color; thus, seven different tubes 

 might contain preparations which, exposed to the sun, or diffused daylight, 

 or the electric light, should yield the Seven rays of the spectrum. The light 

 emitted 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. By 

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

 caused it to give out different colored rays by the single action of one com- 

 mon ray; this variation in power returning to a common degree as the tem- 

 peratures of the phosphori became the same in all. He showed that time 

 Avas occupied in the elevation of the phosphorescent state by the ray, and 

 also that time was concerned in various degrees during the emission of the 

 phosphorescent raj*; 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 spec- 

 trum, pointing out where the maximum effect occurred; also that there were 

 the equivalents of dark bands, i. e. } bands in the spectrum, where little or 

 no phosphorescence was produced. These phosphori were many of them 

 highly fluorescent. Thus, if one of them 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 &ame body seemed to be both phosphorescent 

 and fluorescent. Considering this matter, and all the circumstances regard- 

 ing time, Becquerel was led to believe that these two luminous conditions 

 differed essentiallv onlv 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 that a 

 phosphorescent was only a more sluggish body, which continued to shine 

 after the exciting ray was withdrawn. To investigate this point, he invented 

 the phosjihoroscope, an apparatus which may vary in its particular construc- 

 tion, but in which disks or other surfaces, illuminated by the sun or an elec- 

 tric 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 illumination. By such an apparatus Becquerel showed that all the 

 fluorescent bodies were really phosphorescent, but that the emission of light 

 endured only for a shoit time. An extensive series of experimental illustra- 

 tions upon the foregoing points was made with fine specimens of phosphori, 

 for which the speaker was indebted to M. Becquerel himself. The phospho- 



