160 Professor Faraday, [June 17, 



has impinged upon the substance seen, and has been deflected into a 

 new course, namely towards the eye ; it may be considered as the 

 same ray, both before and after it has met with the visible body. But 

 the light of phosphorescence cannot be so considered, inasmuch as 

 time is introduced ; for the body is visible for a time sensibly after it 

 has been illuminated, which time in some cases rises up to minutes, and 

 perhaps hours. This condition connects these phosphorescent bodies 

 with those which phosphoresce by heat, as apatite and fluor-spar ; for 

 when these are made to glow intensely by a heat far below redness, it 

 is evident that they have acquired a state which has enabled them jfor 

 a time to become original sources of light, just as the other phospho- 

 rescent bodies have by exposure to light acquired a like state. And 

 then again there is this further fact, that as the fluor spar which has 

 been heated, does not phosphoresce a second time when reheated, 

 still it may be restored to its first state by passing the repeated dis- 

 charge of the electric spark over it, as Pearsall has shown. 



Then follows on (in the addition of effect to effect) the phenomena 

 oi fluorescence, and the fine contributions to our knowledge of this 

 part of light by Stokes. If a fluorescent body, as uranium glass, or a 

 solution of sulphate of quinine, or decoction of horse-chestnut bark are 

 exposed to diffuse day-light, they are illuminated, not merely abundantly 

 but peculiarly, for they appear to have a glow of their own ; and this 

 glow does not extend to all parts of the bodies, but is limited to 

 the parts where the rays first enter the substances. Some feeble 

 flames, as that of hydrogen, can produce this glow to a considerable 

 degree. If a deep blue glass be held between the body and the rays 

 of the sun, or of the electric lamp, it seems even to increase the effect ; 

 not that it does so in reality, but that it stops very many of the luminous 

 ray, yet lets the rays producing this effect pass through. By using 

 the solar or electric spectrum, we learn that the most effectual rays are 

 in most cases not the luminous ones, but are in the dark part of the 

 spectrum ; and so the fluorescence appears to be a luminous condition 

 of the substance, produced by dark rays 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 out of the 

 light, or the light shut off from it. 



When E. Becquerel attacked this subject he enlarged it in every 

 direction.* First of all, he prepared most powerful phosphori ; these 

 being chiefly sulphurets of the alkaline earths, strontia, baryta, lime. By 

 treatment and selection he obtained them so that they would emit a 

 special colour : thus, seven different tubes might contain preparations 

 which exposed to the sun, or diffused day-light, or the electric light, 

 should yield the seven rays of the spectrum. The light emitted 



* Annales dc Chimie et de Physique, 1859, tome Iv. p. 1, 



