MEMOIR XXVIIL] DISTRIBUTION OF HEAT IN THE SPECTRUM. 393 



mirror there are therefore three foci. At a distance of 

 eleven inches there is one, e, Fig. 88, giving a spectrum 

 image of the sun. Still farther there is a second,/, which 

 is a spectrum image of the slit, #, in which, if the prism 

 be at its angle of minimum deviation, and the other ad- 

 justments be correctly made, will be seen the Fraunhofer 

 lines. Again, still farther off, at </, is a focal image of 

 the rectangular opening of the black paper, c c, on the 

 front face of the 

 prism. This im- 

 age, arising from 

 the recombination 

 of all the dispersed 

 rays, is consequent- 

 ly white. The sec- 

 ond and third foci 

 are at distances 

 from the mirror de- 

 pending on the dis- 

 tance of the slit, a, 

 and the black pa- 

 per, cc, respectively. 



With the intention of being certain that the light com- 

 ing through the slit, #, is falling properly on the rectangu- 

 lar opening in the prism screen, c c, a small looking-glass 

 is placed at^>. The experimenter, sitting near the mul- 

 tiplier, m, can then see distinctly the reflected image of 

 that opening. 



At the place where the second focal image, with its 

 Fraunhofer lines, forms, two screens of white pasteboard, 

 \ i, are arranged. By suitably placing the former of 

 these, /, the more refrangible rays may be intercepted, 

 and in like manner by the other, i, the less refrangible. 

 In using these screens, and particularly A, care must be 

 taken that no rays passing from the prism to the mirror 



Pig. 87. 



