Prof. Tyndall on Luminous and Obscure Radiation. 333 



Table I. 



Appearance of spiral. Deflection by obscure rays. 



Dark 1 



Dark . 6 



Faint red 104 



Dull red 12*5 



Red 180 



Full red 27*0 



Bright red 44*4 



Nearly white 54*3 



Full white 6O0 



15. The deflection of 60° here obtained is equivalent to 122 

 of the first degrees of the galvanometer. Hence the intensity of 

 the obscure rays in the case of the full white heat is 122 times 

 that of the rays of the same refrangibility emitted by the dark 

 spiral used at the commencement. Or, as the intensity is pro- 

 portional to the square of the amplitude, the height of the sethe- 

 real waves which produced the last deflection was eleven times 

 that of the waves which produced the first. The wave-length, of 

 course, remained the same throughout. 



16. The experimental answer, therefore, to the question above 

 proposed is, that the amplitude of the old waves is augmented 

 by the same accession of temperature that gives birth to the new 

 ones. The case of the obscure rays is, in fact, that of the lumi- 

 nous ones (of the red of the spectrum, for example), which 

 glow with augmented intensity as the temperature of the radiant 

 source is heightened. 



17. In my last memoir* I demonstrated the wonderful trans- 

 parency of the element iodine to the extra-red undulations. A per- 

 fectly opake solution of this substance was obtained by dissolving- 

 it in bisulphide of carbon, and it was shown in the memoir referred 

 to that a quantity of iodine sufficient to quench the light of our 

 most brilliant flames transmitted 99 per cent, of the radiation 

 from a flame of hydrogen. 



18. Fifty experiments on the radiant heat of a hydrogen-flame, 

 recently executed, make the transmission of its rays, through a 

 quantity of iodine which is perfectly opake to light, 



100 per cent. 

 To the radiation from a hydrogen-flame the dissolved iodine is 

 therefore, according to these experiments, perfectly transparent. 



19. It is also sensibly transparent to the radiation from solid 

 bodies heated under incandescence. 



20. It is also sensibly transparent to the obscure rays emitted 

 by luminous bodies. 



* Phil. Trans, vol. cliv. p. 327. [This memoir will appear in the Decem- 

 ber Number of the Philosophical Magazine.] 



