1852.] OF THE ROYAL INSTITUTION. 175 
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 
(u) 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 shewn from Cauchy’s theory that 
for different wave lengths (A) there must be in every medium a 
certain limit of all refrangibility: that is, as we suppose (A) to in- 
crease, large changes in (A) will give continually smaller changes 
in (u), and when (A) is very great compared with (Az) the intervals 
of the molecules, then the index (y) 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 * 
1 
ee B=+Q ( > ful ee )’ — &e., which when we sup- 
pose (“*) = 0 will have for its limiting value (=) =VP 
The results from observation for Rock salt compared with this 
theory are as follows : 
Rock Salt. 
B 
rays 
obs. theory 
mean light 1.558 
red ray 1.540 a 
A = .000079 ays 1.529 
dark hot metal 1.528 sits 
Limit ee 1.527 
But it is to the capital fact established by Professor Forbes, of the 
polarization of heat from dark sources (for with /uminous 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 dipolarization, 
the consequent interferences, the production of circular and elliptic 
vibrations under the proper conditions, — to those familiar with the 
* See the Author’s Treatise ‘“ On the Undulatory Theory applied to the Dis- 
persion of Light,” &c. London, J. W. Parker, 1841, pp. 71-122. 
