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 XLII. Intelligence and Miscellaneous Articles. 



ON THE THERMAL RADIATION PRODUCED BY FLUORESCENCE. 

 BY PROFESSOR VICTOR PIERRE OF PRAGUE. 



r lPHE question has been repeatedly asked whether radiant heat also 

 -*- is produced by fluorescence ; and Studincka (Wien. A/cad. Be?\ 

 xliv. 2. p. 289) and Prince Salm-Horstmar have convinced them- 

 selves that the answer must be affirmative. As regards visible rays, 

 it is virtually superfluous to put the question ; for it is established 

 that by fluorescence such visible rays are formed as are known to 

 produce heating effects ; and (he only question that can arise is, 

 whether by fluorescence obscure rays also are formed — those, that 

 is to say, whose wave-length is greater than that of the extreme 

 red rays. 



This question engaged my attention nine years ago, yet without 

 yielding satisfactory results; my want of success I attributed to the 

 very imperfect apparatus at that time at my disposal. I have 

 since resumed the investigation with better apparatus and in a more 

 favourable locality, and my results form the subject of this paper. 



The best way of deciding the question appeared to be to trans- 

 mit the light of the sun, or of an artificial constant source of light, 

 first through a medium which is transparent, but at the same time 

 opake (or nearly so) to obscure rays ; the transmitted rays are then 

 to be received on the free surface of a fluorescent substance, and the 

 thermoscopic action of the reflected rays measured. Inverting, then, 

 the process by allowing the light first to act directly on the fluores- 

 cent substance, and then, before it impinges on the thermoscope, 

 passing it through the opake medium, there ought to be a diminu- 

 tion of the thermal action in the second case, provided obscure rays 

 were formed by fluorescence. 



If s is the quantity of luminous rays, and d that of the obscure 

 rays in incident light, the quantity of rays emerging from the inter- 

 posed plate are as and (3d. By reflexion these rays are changed 

 into pecs and p'[3d. By fluorescence, a new quantity of luminous 

 and obscure rays are formed — by the radiation as the quantity fas 

 of luminous and;/"asof obscure rays, by the radiation fid the quan- 

 tity f'fid of obscure rays. Hence the total radiation acting on the 

 thermoscope is 



M = as(p'+f") + fid( P ' +f')+f'as. 



If the light falls directly on the fluorescent substance, there is ob- 

 tained in reflected light the quantity of rays ps and p'd, owing to 

 mere reflexion ; by fluorescence there are added the quantities fs of 

 luminous and jf"s of obscure rays from the incident radiation s, and, 

 further, the quantity f'd of obscure rays from the radiation d. If 

 these rays pass through the same plate as before, the quantities 

 emerging are as{p-\-f) of visible, and fid(p' -\-f")-\-pf's of obscure 

 rays ; so that the quantity of rays acting on the thermoscope will be 



M'=*s(p+f)+fid(p'+f")+Pf' S . 



