10 PEOFESSOE TYNDALL ON CALOEESCENCE. 



constructed whose parallel faces were 2-3 inches apart, and which, when filled with the 



solution of iodine, allowed no trace of the most highly concentrated luminous beam to 



pass through it. Five pairs of experiments executed with this cell yielded the following 



results : — 



Kadiation from Electric Light ; battery 40 cells. 



Deflection, 

 o o 



fThrough transparent bisulphide . 47'0; 46-0 



IThrough opaque solution . . . 42-3; 43'5 



fThrough transparent bisulphide . 44-0 ; 43*7 



LThrough opaque solution . . . 41-2 ; 40-0 



Through transparent bisulphide . 42-0; 43*0 



Calling the transmission through the transparent liquid 100, and taking the mean of 

 all these determinations, the transmission through the opaque solution is found by 

 calculation to be 86-8. An absorption of 13'2 per cent, is therefore to be set down to 

 the iodine. This was the result with a battery of forty cells ; subsequent experiments 

 with a battery of fifty cells made the transmission 89 and the absorption 11. 



Considering the transparency of the iodine for heat emitted by all sources heated up 

 to incandescence, as exhibited in Table III., it may be inferred that the above absorp- 

 tion of 11 per cent, represents the calorific intensity of the luminous rays alone. By the 

 method of filtering, therefore, we make the invisible radiation of the electric light eight 

 times the visible. Computing, by means of a proper scale, the area of the spaces 

 A B C D, C D E (fig. 3), the former, which represents the invisible emission, is found 

 to be 7*7 times the latter. Prismatic analysis, therefore, and the method of filtering 

 yield almost exactly the same result. 



§4. 



In the combination of bisulphide of carbon and iodine we find a means of filtering the 

 composite radiation from any luminous source. The solvent is practically transparent, 

 while the dissolved iodine cuts off every visible ray, its absorptive power ceasing with 

 extraordinary suddenness at the extreme red of the spectrum. Doubtless the absorption 

 extends a little way beyond the red, and ynih a very great thickness of solution the 

 absorption of the extra-red rays might become very sensible. But the solution may be 

 employed in layers which, while competent to intercept every trace of light, allow the 

 invisible calorific rays to pass with scarcely sensible diminution. 



The ray-filter here described was first publicly employed in the early part of 1862*. 

 Concentrating by large glass lenses the radiation of the electric lamp, I cut off the 

 visible portion of the radiation by the solution of iodine, and thus formed invisible foci 

 of an intensity at that time unparalleled. In the autumn of 1864 similar experiments 

 were executed witli rock-salt lenses and with mirrors. The paper " On Luminous and 

 Obscure Radiation," already referred to, contains an account of various effects of com- 



• Philosophical Transactions, 1862, p. 67, note. 



