24 Prof. Powell’s further Observations on M. Cauchy’s 
and, instead of it, I have maintained the simple conclusion of 
two kinds of heat simultaneously emanating or originating 
from luminous hot bodies. 
As to contending that this distinction of two kinds of heat 
‘‘ suffices to explain all the facts relative to transmission,” 
I should have been glad if M. Melloni had pointed out any 
passage in which I have contended for anything of the kind. 
The facts of transmission (and for all these most curious and 
important facts we are entirely indebted to the experimental 
skill of M. Melloni) are of a kind as yet appearing so little 
reducible to fixed laws that I should imagine any theory en- 
tirely premature; certainly I have offered none. 
The experiment described by M. Melloni (p. 477 at the 
bottom) is undoubtedly a most curious and interesting one, 
but how it applies to the question relative to mine I fail in 
perceiving. It proves, the author conceives, that there are in 
this case ‘several different kinds of dark heat.” It is true we 
have hitherto known of but one, and I have referred only to 
one in my researches, but I have never denied that there may 
be two, three, or a hundred kinds. I have merely maintained 
that there are characteristic and well-marked distinctions in 
the properties of any or all non-luminous heat (to adopt for 
brevity the barbarously incorrect language which is becoming 
current) from those of the luminous kind; but there may still 
be many more such characteristic distinctions, and some such 
M. Melloni seems to have established in this experiment. 
I look with great interest to the further extension of this 
curious inquiry on a point requiring the most careful exami- 
nation; while I acknowledge that the thermomultiplier of 
M. Melloni has opened to us an entirely new field of investi- 
gation, and in the hands of its inventor and of Prof. Forbes 
has done more for the advance of our knowledge in this de- 
partment within a very short time past, than the most san- 
guine would have ventured to anticipate. 
VIII. Further Observations on M. Cauchy’s Theory of the Di- 
spersion of Light. By the Rev. BapEN Powe 1, M.4A., F.R.S., 
Savilian Professor of Geometry, Ouford.* 
N a former paper, which is inserted in successive portions of 
this Journal, No. 31 e¢ seq. (vol. vi.), I have given an abstract 
of M. Cauchy’s highly important researches on the undulatory 
theory, so far as they bear on the great question of the disper- 
sion of light, and in conclusion have deduced a simple for- 
* Communicated by the Author: on this subject see also a preceding 
article by Mr. Tovey. 
