658 
Temperature of the Sun. 
[October, 
radiating at least as much heat as a square foot of the 
metal ; for it is obvious that the distances of the two 
sources have nothing to do with this effect under the given 
conditions. 
The metal area, however, being many times that of the 
sun, the latter still over-balanced the metal ; showing that 
the sun was actually very much the hotter. Accordingly, 
there was interposed between the porte-lumiere and the pile a 
telescope which diffused the sun-light over an image of any 
given diameter. As the solar light entered only through a 
diaphragm of known dimensions, it was easy to say how 
much the sun's heat was weakened to balance that from 
the metal. It must be borne in mind, however, that there 
was no account taken of the loss of solar heat by reflection 
and absorption in the lenses, by reflection from the mirror, 
and more than all by the frequent clouds of smoke and 
steam, while the furnace heat suffered no diminution what- 
ever. Further, every other condition of the experiment was 
designedly such as to weigh in favour of the furnace and 
against the sun’s heat. The value found for the latter, 
then, is a minimum value. I should, perhaps, have 
remarked that experiments had shown that the trifling heat 
from objects near the melted metal might be negleCted. 
That from the atmosphere about the sun was also insig- 
nificant. Except, then, for the diminution of solar heat by 
absorption, reflection, and so on, our method is equivalent 
to bringing a specimen piece from the sun’s surface (if I may 
so express myself) face to face with one from the furnace, 
placing our thermopile mid-way between them, and deter- 
mining how much we have to diminish the size of the 
former to make its heat-radiation no more than equal the 
latter’s. 
The result of these experiments was that the minimum 
value we can assign to the solar radiation is eighty-seven 
times that from an equal area of the pouring metal. This, 
it will be remembered, is not an aCtual but a minimum 
value. The true value may be indefinitely greater. 
Photometric Comparisons. 
Of the complex radiations from any source of high tem- 
perature, a part is interpreted by the pile as heat, a part 
by the eye as light ; but as the temperature is raised, it is 
now well known that the waves of shorter length increase 
in amplitude much faster than the longer ones. If the 
temperature of the sun, then, be much greater than that of 
