Prof. Tyndall on the History of Negative Fluorescence. 49 



may or may not be " rigid ;" but, at all events, they are not the 

 experiments and the reasoning of Dr. Akin. The intelligent 

 reader will now form his own estimate of the gravity of my 

 offence when I ventured to put myself forward as an humble 

 corroborator, by reasoning of my own, of views which I stated to 

 have been previously enunciated by another. 



With regard to my reading habits, I would simply say that it 

 is hardly becoming on the part of a gentleman in his position to 

 lay down the law, on this head, for the hardworking experimenter. 

 Dr. Akin has, thus far, done little but read and write. I do 

 not object to this, but I do object to his compelling me to adopt 

 his habits. As a matter of principle, I reduce my reading to a 

 minimum ; and high as my opinion is of the functions of the 

 British Association, I do not pay strict attention to the news- 

 paper reports of its proceedings. This accounts for the fact that, 

 until Dr. Akin drew my attention to it, I was not aware of the 

 existence of the passage which he has cited from the 'Athenaeum.' 

 That passage, however, imposes no new duty upon me ; I have 

 already explicitly recognized the priority of Dr. Akin with regard 

 to the point in question. 



But the reader of the " Note on Ray-transmutation " will 

 have already surmised that the seat of its author's discontent lies 

 far deeper than this question of a platinum wire and a hydrogen- 

 flame. Let us now inquire into his relationship to the real pro- 

 blem, which may be broadly stated thus : — 



To raise the refrangibility of invisible rays of long period, so as 

 to convert them into visible rays. 



This problem would be solved by raising an incombustible 

 body to a state of incandescence, by perfectly invisible rays of low 

 refrangibility. The consideration of the problem at once limits 

 us to those obscure rays of great intensity which are known to 

 be emitted from highly luminous sources; for it would never 

 occur to any one practically acquainted with the subject, to 

 attempt to produce incandescence by rays emanating from an 

 obscure source. The field of experiment is thus narrowed at the 

 outset, and I am only acquainted with two sources which offer 

 any prospect of success. These are the sun and the electric light. 

 The obscure radiation of the sun was established in the year 

 1800 by Sir William Herschel, who then proved by far the 

 greater portion of the sun's thermal power to be due to invisible 

 rays of low refrangibility. I am not aware whether anybody but 

 myself has worked at the invisible radiation of the electric light, 

 but I may say that this subject has occupied me, at frequent 

 intervals, during the last ten years. 



Limited thus to rays emitted by luminous sources, the two 

 obvious conditions of experiment are the suitable concentration 



Phil. Mag. S. 4. Vol. 29. No. 193. Jan. 1865. E 



