368 RADIANT HEAT. 



satisfactory manner. It would be also highly important to make 

 similar observations on the oxy-hydrogen and the electric light. 



The theory of unequal wave-lengths, as the sole explanation of the 

 "different species of "radiant heat,' ; whether solar or terrestrial, or in 

 other words, the identity of the rays which produce alike the sensa- 

 tions of light, of heat, or other effects, each in some peculiar relation 

 to the wave-length, certainly applies in a very satisfactory manner to 

 a large portion of the phenomena. There may, indeed, be some 

 minor objections or difficulties, but the only formidable outstanding 

 objection seems to arise from a single result, announced long ago by 

 Melloni, and referred to in my former report, viz: the fact that a cer- 

 tain kind of green glass transmits the solar light in high intensity 

 while it deprives it of all heating power. This anomaly is indeed in 

 itself so singular as to require very positive authority to substantiate 

 it; and in M. Melloni' s statement there is, as appears to me, a certain 

 degree of vagueness, and it is not supported by any numerical results, 

 or even any detailed account of the mode of operating"'. 



An alleged isolated fact of so extraordinary a character has long 

 appeared to me to demand a strict re-examination. I had hoped that 

 on presenting this report, in many other respects so imperfect, I 

 might have been able to announce the result of such a repetition. 

 But, unhappily, a variety of causes have hitherto prevented me from 

 carrying it into effect. 



Theoretical Refraction of Heat. 



An important point bearing on the theory was indicated very 

 shortly after the communication of my last report, (1840,) in my trea- 

 tise " On the Undulatory Theory applied to Dispersion," (1841.) I 

 have there shown (see pp. 71 and 122) that the formula for the re- 

 fractive index in terms of the wave-length deduced from Cauchy's 

 theory, furnishes a striking coincidence with Professor Forbes' s de- 

 termination of the index of refraction for a ray of dark heat in rock- 

 salt. The formula in question way be expressed thus: 



^(SMr)'-**- 



F 



where ft is the index, X the wave-length, Ax the small interval of the 



molecules, and P, Q, R constants. From the nature of this formula, 



it is evident that as X is increased the changes corresponding in the value 



Ax 

 of p. are very small; and when l is very great, or — extremely small, 



the value of fj. is susceptible of a limit, which will be 



1 



This will represent the physical condition, that as we take rays of 

 successively greater wave-length, they will be crowded together into 



5 On the Identity of Light and Heat, Taylor's Foreign Memoirs, Part III, p. 38S. 



