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VIII. On the History of Negative Fluorescence. 

 By John Tyndall, F.R.S. $c* 



A COMMUNICATION from the pen of Dr. Akin, pub- 

 lished in the last Number of the Philosophical Magazine, 

 will, I trust, be my excuse for giving the following brief sketch 

 of my relation to the question of " negative fluorescence " — a 

 term which may be provisionally taken to express the changing 

 of the refrangibility of the invisible ultra-red rays of the spec- 

 trum so as to render them visiblef. 



In the month of June 1852, a scientific gentleman, eminent 

 though young, showed me, in his lodgings in Duke Street, Pic- 

 cadilly, the fluorescence of an infusion of horsechestnut bark, 

 and explained to me the nature of the discovery of Prof. Stokes. 



In the month of September of the same year, I heard Prof. 

 Stokes lecture on this subject before the Belfast Meeting of the 

 British Association. At that Meeting, moreover, a term was 

 employed by the gentleman who had first enlightened me on the 

 subject of fluorescence, which I have never since forgotten. He 

 said that the light was always degraded (meaning thereby that its 

 refrangibility was always lowered) when fluorescence was exhibited. 



This lowering of the refrangibility was one of the most promi- 

 nent characteristics of the phenomena brought to light by Prof. 

 Stokes, and it inevitably provoked the opposite question. In 

 common, I believe, with many of those who had heard the ex- 

 periments described, I soon afterwards inquired whether it was 

 not possible to elevate refrangibility, and thus to render the ultra- 

 red rays of the spectrum visible, as Prof. Stokes had rendered the 

 ultra-violet ones. 



I believe I am right in saying that though, owing to the total 

 absence of facts, nothing was published on the subject, the ques- 

 tion of a change of refrangibility, in an upward direction, was not 

 an uncommon topic of conversation among scientific men. In the 

 year 1859, moreover, the writer already quoted, and who is also 

 referred to by Dr. Akin, wrote as follows : — ■" The thought occurs 



* Communicated by the Author. 



t This term was introduced by M. Emsmann in 1861, in the following 

 words : — " Starting from the fact that, in the phenomena of fluorescence 

 hitherto observed, a lowering of the refrangibility or an augmentation of the 

 wave-length occurs — a law which can by no means be regarded as firmly 

 established — we cannot deny all justification to the opinion that a kind of 

 fluorescence must also exist, the essence of which would consist in an ele- 

 vation of the refrangibility or a diminution of the wave-length. The former 

 kind of fluorescence would be that of the chemical rays, which I would 

 propose to call positive fluorescence ; the latter that of the calorific rays, 

 which I would call negative fluorescence " (Pogg. Ann. vol. cxiv. p. 652). In 

 1863 Dr. Akin introduced the term calcescence to express the same thing. 



