168 &. W. Draper—Distribution of Heat in the Spectrum. 
this mirror there are therefore three foci. Ata distance of eleven 
inches there is one, ¢, fig. 3, giving a spectrum image of the sun. 
Still further there is a second, f, which is a spectrum image of the 
slit a, in which, if the prism be at its angle of minimum devia- 
tion, and the other adjustments be correctly made, will be seen 
the Fraunhofer lines. Again, still farther off, at g, is a focal 
image of the rectangular opening of the black paper ¢ ¢, on the 
Fig. 2. 
front face of the prism. This image, arising from the recombin- 
ation of all the dispersed rays, is consequently white. The 
second and third foci are at distances from the mirror depending 
on the distance of the slit a, and the black paper ¢ ¢, respect 
ively. 
With the intention of being certain that the light coming 
through the slit @ is falling properly on the rectangular opening 
in the prism screen ¢c,a small looking-glass is placed at p. he 
experimenter, sitting near the multiplier m, can then see Gi 
tinctly the reflected image of that opening. 
t the place where the second focal image with its Fraut- 
hofer lines forms, two screens of white paste- , A, % are 
arranged. By suitably placing the former of these, /, the more 
refrangible rays may be intercepted, and in like manner by the 
