i879-J 
The Temperature of the Sun. 
659 
the furnace, we shall have a quite independent proof of the 
fadt in a photometric comparison, which, we can safely 
pronounce a priori , will then give a very much greater ratio 
of sunlight to furnace-light than that of sun-heat to furnace- 
heat. To make this comparison, a photometer box, about 
8 inches in square section and 66 inches in length, is placed 
so that its central axis lies as before in the path of the 
reflected beam from the mirror to the furnace. Two similar 
telescopes of r*66 inch aperture and 20*01 inches focus, 
having their objectives outside the extremities of the box 
and their optical axes in the path of the beam, project, by 
their eye-pieces, images of the sun and of the pouring metal 
on the two sides of a Bunsen disk, whose normal position is 
in the centre of the box. Both images are viewed simul- 
taneously by mirrors attached to the disk, which is movable 
along a graduated scale. (I here omit certain small cor- 
rections applied in practice, and describe the use of the 
instrument in brief terms.) We do not now need to consider 
the relative angular areas of the sun and furnace, for so long 
as both are of appreciable size the images of both falling on 
the screen, when nearly midway between the two telescopes, 
will be sensibly proportioned in brightness to the absolute 
intensities of the sun-light and furnace-light. We do, in fadt, 
however, at the outset find the sun-light so immensely 
brighter that no direcSt comparison is possible. We then 
diminish the aperture of the solar telescope (which we will 
call A), till it has a small known ratio to that of the furnace 
telescope (which we will call B). In practice B was always 
left with the full aperture of i*66 inch diameter, while that 
of A was 0*192. Were the original sources of equal intensity, 
the sun-light would have been reduced to =0*013 + 
or a little over one one-hundredth of the other. But it was 
surprising to see that the image from A was even now 
incomparably stronger than that formed either by the flame 
from the blast at its brightest, or by the pouring metal. 
Under these circumstances, the Bunsen disk was moved 
from its central position toward B, thus approaching the 
apex of one light cone and withdrawing from the other, so 
as to diminish the sun-light still further in an exactly deter- 
minable ratio. The lowest value obtained in a series of 
accordant measures gave intensity of sun-light over (5300) 
five thousand and three hundred times that from the 
metal ; and this value is, I think, considerably below the 
truth. 
It results from these experiments : (1) That diredt observa- 
