110 Prof. C. Bohn on Negative Fluorescence 



one or two minutes, became luminous — that is, gave out rays of 

 higher refrangibility than those that it received, I mentioned 

 also that when the crystal did not hang free in the air but was 

 enclosed in a glass tube and put into the heated space, it did 

 not become luminous till after about fourteen or eighteen minutes. 

 As glass is known to exert very great absorptive action on dark 

 rays of heat, I attributed the luminosity in this case to the heat 

 received by the mineral through contact with the heated sides 

 of the glass tube. I did not, however, neglect to state, on the 

 occasion of making this communication, that the phenomenon 

 which I had observed, as well as the phenomena of incandes- 

 cence which Prof. Tyndall had produced at the focus of invisible 

 rays of heat, and the incandescence of a platinum wire or piece 

 of lime under the action of a hydrogen -flame, which of itself 

 gives out almost exclusively dark rays of heat, differed from the 

 phenomenon of fluorescence proper in a not unessential respect, 

 inasmuch as the latter begin instantly the fluorescent substance 

 is exposed to the exciting rays, whereas the former do not make 

 t heir appearance until considerably later, in fact not until the 

 body which exhibits them has become very hot. And I further 

 pointed out that whereas fluorescence continues as long as the 

 fluorescent body is exposed to the exciting rays, the phospho- 

 rescence of fluor-spar soon diminishes in intensity and at last 

 ceases entirely, although it be continuously exposed to radiant heat. 

 I supposed at that time that the phosphorescence of fluor- 

 spar would begin as soon in a space surrounded by plates of 

 rock-salt as when it was separated from the radiating sides of 

 the stove only by air, as was the case in one of my experiments, 

 since rock-salt does not absorb rays of heat to a much greater 

 extent than air does. I gave expression to this opinion (only in 

 a letter, it is true) as early as March 1866, and suggested to a 

 well-known physicist, who had plates of rock-salt at his disposal, 

 to test my conclusion by experiment. So far as I am aware, 

 this has not been done. In the meantime I have succeeded in 

 obtaining a proper vessel of rock-salt, and have made the expe- 

 riment myself. I have been thereby convinced that the phos- 

 phorescence of fluor-spar cannot be regarded in any sense as a 

 phenomenon of negative fluorescence as understood by Ems- 

 rnann (that is, as a direct transformation of rays of lower into 

 rays of higher refrangibility), and that the experiments instituted 

 and discussed by Messrs. Akin and Tyndall do not, any more 

 than the facts cited by M. Emsmann, establish the fact of an 

 increase in the refrangibility of rays*. I have become firmly 



* In connexion with M. Emsmann's views, see M. Hagenbach's remarks in 

 the Berlin Report on the Progress of Physics in 1861 (Fortschritte d. Physik, 

 1861, p. 270). 



