170 soo. ;W. Draper—Distribution of Heat in the Spectrum. 
much greater width than the slit a, on account of the obliquity | 
of the front face of the prism. 
y removing the screen h, and placing the screen 7 so that 
its edge coincides with the line A of the Fraunhofer spectrum, all 
the invisible heat radiations of less refrangibility than the red 
are cut off, except the contaminating ones arising from the gen- 
eral diffusion of light by the substance of the prism. Under 
these circumstances the image on the pile will be white, and 
the multiplier will give a deflection representing the heat of the 
visible and the extra violet regions. If then the screen be 
advanced still further, until it has intercepted all the less 
refrangible regions up to the sodium line D or a little beyond, 
that is, to the optical center of the spectrum, the tint on the face 
of the pile will be greenish-blue, and the multiplier will give a 
measure of the heat of the more refrangible half of the visible 
spectrum, together with that of the ultra-violet rays; the latter 
portion may, however, be eliminated by properly using the 
other screen h. 
esides the error arising from stray heat diffused through the 
spectrum, in consequence of the optical imperfection of the 
prism, there is another which may be recognised on recollecting 
the relative positions of the prism, the concave mirror, and the 
face of the pile. It is evident that the prism, considered as 4 
warm or a cool mass, is a source of disturbance, for the mirror 
reflects its image, that is, the image of the prism itself, to the 
pile. After the intromitted sunbeam has passed through the 
prism for a short time, the temperature of that mass has risen, 
and the heat from this source has become intermingled with the 
proper spectrum heat. But this error is very easily eliminated. 
It is only necessary to puta screen n in the path of the incoming 
ray, between the slit and the prism, and note the deflection of 
the multiplier. Used as we are here supposing, the multipher 
as two zeros. The first, which may be termed the magnetie, 
is the position in which the needles will stand when no current 
is passing through the coil. The scale of the instrument should 
be set to this. The other, which may be termed the working 
zero, is formed by coupling the pile and the multiplier together, 
and introducing the screen n between the intromitting slit a 
the prism. On doing this it will probably be found that the 
index will deviate a few divisions. Its position should be. 
accurately marked at the beginning and close of each 
measures, and the proper correction for them made. Th 
turbing influences of the mass of the prism, of the mirror, and 
of the pile itself, are thus eliminated. As respects the last, It 
should not be forgotten that it may be affected by changes 1? 
the position of the person of the experimenter himself. 
'ith the intention of diminishing these errors, I have usually 
