164. Sf. W. Draper—Distribution of Heat in the Spectrum. 
wave-lengths. It thus appears that in the prismatic, from the 
fixed line p to A, the yellow, orange, and red regions occupy 
but little more than half the space they do in the diffraction; 
while the green, blue, indigo, and violet, from the fixed line E 
to H, occupy nearly double the space in the prismatic that they 
do in the diffraction spectrum. The general result is that in 
the prismatic the less refrangible regions are much compressed, 
and the more refrangible much dilated. And it is plain that 
the same will hold good in a still greater degree for any invist- 
pemye that are below the red and above the violet respect- 
ively. 
an increased heat for that region; and on the contrary, the dila- 
tation of the more refrangible would give an exaggerated dimr 
nution of heat for that space. But if it were possible to make 
satisfactory heat measures on the diffraction spectrum, im which 
the colored spaces and fixed lines are arranged according t0 
their wave-lengths, the admission would be substantiated. 
In view of these facts I did attempt many years ago to make 
heat measures on the diffraction spectrum. But so small is the 
heat that, as may be seen in the Philosophical Magazine (Mareh, 
1857), the results were unsatisfactory. More recently I have 
tried another method of investigation, on principles which I 
will now explain. 
For the sake of clearness, restricting our thoughts for the 
moment to the more familiar case of the visible spectrum, if we 
desired to ascertain the true distribution of heat, would not the 
proper method be to collect all the more refrangible rays into 
one focal group, and all the less refrangible into another foca 
group, and then measure the heat that each gave? If the view 
currently received be correct, would not nearly all the heat 
observed be found in the latter of these foci, and little, if 
indeed any, be found in the former? But if all the various 
regions of the spectrum possess equal heat-giving powers, WO 
not the heat in each of these foci be the same ay 
Let us give greater precision to this idea. Using Angstrom® 
