40 



Dr. C. K. Akin on Calcescence. 



could not be wholly relied upon, as regards its absolute imper- 

 viousness to rays of higher refrangibility than the line D*. As 

 regards black glass, Melloni has published the following numbers, 

 showing its relative permeability to rays from different sources, 

 the incident rays being reckoned as lOOf : — 





Thickness. 



Alcohol- 

 flame. 



Incandescent 

 platinum. 



Oil-flame. 



Colourless glass ... 

 Black glass 



millim. 

 0-88 

 0-62 

 1-84 



41-2 

 52-6 

 29-9 



52-8 

 42-8 

 271 



70-6 

 37-9 

 25-3 



Black glass 



M. A. Matthiessen of Altona also has published observations, 

 made on the solar spectrum, which prove the permeability of 

 opake black glass (manufactured at Choisy-le-Roi) to the invisible 

 Herschellic raysj. I consequently tried to procure such black 

 glass both from London and Paris, but only towards the end of last 

 summer I succeeded in getting a specimen from Paris, through 

 Mr. Becker; the absorbing and transmitting powers I have not yet 

 been able to test. Finally, as regards the solution of iodine in 

 bisulphide of carbon, my attention was directed to it by a passage 

 in the work on Heat, by Prof. Tyndall, who has recently pub- 

 lished further experiments on the subject. Considering, however, 

 that the iodine solution must always be placed between glass 

 sheets, and that the transparencies of white and black glass for 

 Herschellic rays seem to be nearly equal, whilst extremely thin 

 sheets of black glass seem capable of excluding the light of the 

 most powerful sources §, perhaps black glass, upon the whole, will 

 appear preferable as a diaphragm to the solution of iodine. 



Another species of diaphragui required, and to be used as an 



* However, in no case is it possible to be sure that a given radiation is 

 totally absent ; all that it is possible to aver is that it is imperceptible. 

 For this reason it is perhaps equally decisive to prove, in such experiments 

 as those that were to be instituted by means of the above red glass, that 

 the radiation emitted is of greater intensity than the incident, as to prove 

 that the radiation emitted is perceptible, whilst in the incident beam the 

 same radiation is apparently wanting — that is to sa}', possibly only imper- 

 ceptible. It would be necessary, however, in the former case not merely to 

 consider the momentary intensity, but to compare the energy represented 

 by the incident rays with the whole of the energy represented by the cor- 

 responding emitted rays, as long as the emission lasts. 



t La Thermochrose, p. 178. Compare also the coresponding data in 

 pp. 227, 265, 295, 298, 309, and 310. 



X Comptes Rendus, vol. xvi. p. 7G3 (1843). 



§ According to Melloni, black glass of the kind employed in the experi- 

 ments above quoted (" such as opticians employ for the construction of 

 polarizing mirrors") is competent to intercept the light of "the most 

 brilliant sunshine " in thicknesses as small as 0*596 millim. (La Thermo- 

 chrose, pp. 289 and 310.) 



