and Phosphorescence. 109 



1859 M. Emsmann* suggested that it might be possible to de- 

 tect cases of altered refrangibility of rays in which the change 

 consisted in an increase, instead of in a diminution, of refrangi- 

 bility. He proposed to call the already well-known phenome- 

 non of a decrease of refrangibility "Positive Fluorescence;" 

 and the converse phenomenon of an increase of refrangibility, 

 which he regarded as possible or probable, he proposed to call 

 " Negative Fluorescence." In the year 1861f he cited certain 

 familiar facts (such as changes of colour consequent on elevation 

 of temperature) as tending to establish the existence of this ne- 

 gative fluorescence; and he has recently J asserted that these 

 phenomena afford an actual proof of such a transformation of rays. 

 Attempts have lately been made to detect experimentally the 

 transformation of rays of heat into rays of light in accordance 

 with negative fluorescence. Mr. Akin§ and Professor Tyndall|| 

 consider that they have discovered and pointed out distinct phe- 

 nomena of this kind. They have entered into an animated dis- 

 pute, and are disagreed as to which of them was the first to 

 make the discovery ; they disagree also about the name by which 

 the phenomenon in question ought to be denoted (Akin calls it 

 " calcescence " and Tyndall u calorescence "), and are at one only 

 in so far as they both consider that the phenomena of incan- 

 descence which they have either actually produced, or at least 

 proposed to make the subjects of experiment, ought to be re- 

 garded as phenomena of the same kind as those of (positive) 

 fluorescence — that is, as cases of " ray-transformation." In an 

 abstract of the researches that have just been mentioned, which 

 I gave in the Giessen "Annual Report"^]", I remarked that 

 the transformation of rays of lower refrangibility into rays of 

 greater refrangibility could be brought about much more simply 

 than by the methods employed by Mr. Akin and Professor 

 Tyndall. I mentioned that I had suspended fluor-spar freely 

 by means of fine threads in the oven of a common house-stove, 

 so that the mineral w r as exposed to the dark (slightly refran- 

 gible) rays of heat given out by the moderately heated sides of 

 the stove, and that under these circumstances the fluor-spar, after 



* Marbacli's Physikalisches Lexikon, 2nd edit, by Cornelius, vol. vi. p. 

 1081. 



t Poggendorff's Annalen, vol. cxiv. p. 65 1. 



% Ibid. vol. cxxix. p. 352. 



§ The Reader, Sept. 26, 1863. Phil. Mag. S. 4. vol. xxviii. p. 554; 

 vol. xxix. pp. 28 & 136. 



|| Phil. Mag. S. 4. vol. xxviii. p. 32.9. N. Arch. Ph. Nat. vol. xxii. p. 41. 

 Pogg. Ann. vol. cxxiv. p. 36. Phil. Mag. S. 4. vol. xxix. p. 41. Proc. Roy. 

 Soc. vol. xiv. p. 33. Phil. Mag. S. 4. vol. xxix. p. 218. N. Arch. Ph. Nat. 

 vol. xxii. p. 133. Proc. Roy. Soc. vol. xiv. p. 476. Phil. Mag. S. 4. vol. 

 xxix. p. 241. Phil. Trans, vol. lxx. p. 1. 



5[ Jahresbericht iiber die Fortschr. der Chemie, &c. 1865, p. 80. 



