366 
Proceedings of the Royal Society 
3. On a Form of Eadiation Diagram. 
By George Forbes, Esq. 
The following facts appear to have been conclusively established 
by universal experience : — 
1. Nearly all, if not all, solid substances become self-luminous at 
the same temperature. 
2. The red rays are the first to become visible, and, on increasing 
the temperature, colours of less wave-lengths are successively added 
in the order of their wave-lengths. 
3. While colours of shorter wave-lengths are being added, those 
which were previously visible become more intense. 
It appears, then, that the intensity of radiation (i) of any parti- 
cular colour is connected with the temperature 0 , and the wave- 
length A, by some equation 
» =/(*,*•)• 
No data at present exist by means of which the form of this func- 
tion can be determined. Theoretically, however, its determination 
is of great importance, and it also leads to some practical applica- 
tions. For this reason it is worth while making an attempt to 
approximate roughly to a radiation diagram, on which shall be 
drawn curves that are isothermals, the ordinate of any point indi- 
cating the intensity of the radiation of a wave-length indicated by 
the abscissa, at the temperature of the particular isothermal con- 
sidered. 
The experimental data for an exceedingly rough approximation 
to such a curve exist. But difficulties of several kinds are met 
with. 
1. If we judge the intensity of radiation by the eye, as Frauen- 
hofer did, we can only see a limited portion of the spectrum; and 
if we use a thermo-pile, with the face covered with lamp-black, we 
have no proof that all the invisible rays are as thoroughly absorbed 
as we know the visible rays to be. A thermo-pile covered with 
chalk would not absorb the luminous rays so intensely as one coated 
with lamp-black. But we cannot say that lamp-black does not 
