Prof. Tyndall on the History of Negative Fluorescence. 45 



involuntarily, that phenomena analogous to those of fluorescence 

 may be discovered, the explanation of which would have to be 

 referred, not to a lowering, but a raising of the refrangibility. 

 Such would be the case if the ultra- red rays of the spectrum 

 could be rendered visible, as the ultra-violet ones have been"*. 



During the last six years, or since the commencement of my 

 researches upon radiant heat, this subject has been frequently 

 in my thoughts; and I helped myself, whenever opportunity 

 offered, to form conceptions of the physical processes involved 

 in negative fluorescence, by observations upon waves of water. 

 But other questions, of a more pressing character, compelled me 

 to postpone the definite experimental examination of this one. 



Early in the autumn of 1863 I imagined that I had suc- 

 ceeded in proving one of our commonest experiments to be an 

 illustration of a change of refrangibility, in an upward direction. 

 Having, as I conceived, shown the oscillating periods of the mo- 

 lecules in a hydrogen-flame to be ultra-red, I inferred, at once, 

 that the light emitted by a platinum wire plunged into the flame 

 could only be produced by a change of period. It was not, how- 

 ever, a case of real negative fluorescence, for there was no con- 

 version of invisible rays into visible ones. The change of period 

 occurred, as I then phrased it, "before the heat had assumed 

 the radiant form." 



Later on in the year I met, at a Philosophical Club dinner, a 

 gentleman whose own experiments had given him the right to speak 

 with authority in these matters, and I took my place beside him 

 for the express purpose of learning his matured opinion as to the 

 possibility of converting long waves into short ones. I men- 

 tioned to him the observations which I had been making from 

 time to time on sea-waves in the Isle of Wight, and I then 

 first learned that a gentleman named Akin was trying experiments 

 on this subject. 



The remarks made on this occasion were restricted to the con- 

 version of radiant heat of slow period into heat of more rapid 

 period. What I then conceived to be my illustration of conver- 

 sion (namely, the change of the slow periods of a hydrogen-flame 

 into the rapid periods of a platinum wire plunged into the flame) 

 was not referred to. About this time 3 however, the subject was 

 one of those on which I freely conversed with my more intimate 

 scientific friends. To Dr. Debus, for example, I expounded, 

 early in December 1863, the whole question from beginning 

 to end. 



On the 23rd of January, a clever and interesting article, 

 entitled " Calcescence/" was published in the ' Saturday Review/ 

 The author, Dr. Akin, first proposed an entirely new termi- 

 * Marbach's Physikal. Lexicon, vol. vi. (1859) p. 1081. 



