Prof. Tyndall on Luminous and Obscure Radiation. 335 



24. To the luminous rays from the intensely white spiral the 

 solution was perfectly opake; but though by the introduction 

 of such rays the transmission, as expressed in parts of the total 

 radiation, was diminished, the quantity absolutely transmitted 

 was enormously increased. The value of the last deflection is 

 440 times that of the first ; by raising therefore the platinum 

 spiral from darkness to whiteness, we augment the intensity of 

 the obscure rays which it emits in the ratio of 1 : 440. 



25. A rock-salt cell filled with the transparent bisulphide of 

 carbon was placed in front of the camera which contained the 

 platinum spiral raised to a dazzling white heat. The trans- 

 parent liquid was then drawn oif and its place supplied by the 

 solution of iodine. The deflections observed in the respective 

 cases are as follows : — 



Radiation from White-hot Platinum. 

 Through transparent CS 2 . Through opake solution. 



73-9 73-0 



73-8 72-9 



All the luminous rays passed through the transparent bisulphide, 

 none of them passed through the solution of iodine. Still we 

 see what a small difference is produced by their withdrawal. 

 The actual proportion of luminous to obscure, as calculated from 

 the above observations, may be thus expressed : — 



26. Dividing the radiation from a platinum wire raised to a 

 dazzling whiteness by an electric current into twenty-four equal 

 parts, one of these parts is luminous and twenty-three obscure. 



27. A bright gas-flame was substituted for the platinum 

 spiral, the top and bottom of the flame were shut off, and its 

 most brilliant portion chosen as the source of rays. The result 

 of forty experiments with this source may be thus expressed :— 



28. Dividing the radiation from the most brilliant portion of 

 a flame of coal-gas into twenty-five equal parts, one of those parts 

 is luminous and twenty-four obscure. 



29. I next examined the ratio of obscure to luminous rays in 

 the electric light. A battery of fifty cells was employed, and 

 the rock-salt lens was used to render the rays from the coal 

 points parallel. To prevent the deflection from reaching an 

 inconvenient magnitude, the parallel rays were caused to issue 

 from a circular aperture 01 of an inch in diameter, and were 

 sent alternately through the transparent bisulphide and through 

 the opake solution. It is not easy to obtain perfect steadiness 

 on the part of the electric light ; but three experiments carefully 

 executed gave the following deflections : — 



