538 



Royal Institution. 



hands admitted, viz. that the illuminating and heating powers follow 

 very different laws with relation to the different rays. 



The grand discovery by Melloni of the true refraction of heat, 

 even of that kind which constitutes the whole radiation from dark 

 sources, by means of the rock-salt lens and prism, and its extension 

 by Professor Forbes to the determination of the index of refraction 

 (fi) for the most heating rays from all sources, both luminous and 

 non-luminous, gave the first actual proof of the real analogy of the 

 propagation of heat by waves in an etherial medium : which was 

 further carried out when it was shown from Cauchy's theory that 

 for different wave-lengths (X) there must be in every medium a 

 certain limit of all refrangibility : that is, as we suppose (X) to in- 

 crease, large changes in (X) will give continually smaller changes in 

 (fi), and when(X) is very great compared with (A,z) the intervals of 

 the molecules, then the index (/j.) assumes its limiting value, which 

 is not greatly below that for the extreme red ray, and with this, the 

 index for the lowest heat coincides. 



This is seen directly from the formula • 



*• / 



+ 



■&c, which, when we suppose 



( — \ =0, will have for its limiting value ( - \ = \/p~7 



The results from observation for rock-salt compared with 

 theory, are as follows : — 



Rock- Salt. 



this 



But it is to the capital fact established by Professor Forbes, of the 

 polarization of heat from dark sources (for with luminous sources 

 little doubt could exist), with all its remarkable train of consequences, 

 that the complete analogy with light is seen in the most uninter- 

 rupted point of view ; — the transverse vibrations, the depolarization, 

 the consequent interferences, the production of circular and elliptic 

 vibrations under the proper conditions, — to those familiar with the 

 wave-theory present an irresistible accumulation of proof of the 

 identity of the rays of heat with a succession of waves in an etherial 

 medium ; exhibiting different properties in so?ne dependence on their 

 wave-lengths. 



* See the author's Treatise " On the Undulatory Theory applied to the 

 Dispersion of Light," &c. London, J. W. Parker, 1841, pp. 71-122. 



