112 Prof. Powell’s Postscript on the Dispersion of Light. 
which I believe to. be incontestably true) that the distinction is 
not between Jumznous and obscure sources of heat, but between 
the kind of rays of heat emitted from bodies at different zempe- 
tures; and that the accident (as I may term it) of the bodies 
being at such a temperature that rays of light accompanied 
the rays of heat, has nothing whatever to do with the fact of the 
different transmissibilities of the calorific rays: 1st, Because the 
same difference of transmission exists between sources alto- 
gether obscure; 2nd, Because this difference (between lumin- 
ous and obscure sources) does not exist with reference to some 
bodies, e.g. rock salt; and, thirdly, in bodies emitting light, 
the quantity of heat transmitted is in no way proportional, 
either to the degree of light which accompanies it in the first 
instance, or to the quantity of light which passes through 
along with it. 
In reference to this subject, it is to be observed, that it is 
altogether erroneous to consider “ diathermancy” in the sci- 
ence of heat as analogous to * transparency” in optics ; for that 
property of bodies by which they stop (absorb) or transmit 
rays of a particular refrangibility or colour is the true analogue 
in the latter science. 
I suspect that this necessary distinction escaped Professor 
Powell’s attention when he alluded to M. Melloni’s hypothesis 
as “needless and contrary to all analogy ;” for in this view of 
the subject, the explanation which M. Melloni has given of 
the heat being more abundantly transmitted through succes- 
sive plates (of similar natures) is perfectly analogous to the 
effect of a succession of screens (of the same colour) on com- 
mon light. The “ diathermancy” of rock salt alone appears 
entitled to be compared with “ transparency” as used in optics. 
Stephen’s Green, Dublin, Jan. 9, 1836. 
XXII. On the Theory of Dispersion. By the Rev. B. Powell, 
MA., F.R.S., Sav. Prof. of Geometry, Oxford. 
EARNING that there is not room in this Number for 
the continuation I had proposed of the researches com- 
menced in the last, I am anxious meanwhile to make a 
brief remark on two points referred to in the last Number. 
I. Mr. Tovey in an excellent paper on the formula for di- 
spersion, introduces a most material simplification on M. Cau- 
chy’s process. I allude to this more particularly now, because 
the writer refers to the importance of not omitting the summa- 
tion. He will, I trust, find that the introduction of it as 
discussed in my paper (and still more when the continuation 
appears, ) will produce an entire accordance in our results. 
