]'10 Dr. J. W. Draper on the Distribution of 



Journal, Feb. 1847. A given series of waves of red light im- 

 pinging upon an extinguishing surface will produce a definite 

 amount of heat, and a similar series of violet waves should pro- 

 duce the same amount ; for though an undulation of the latter 

 may have only half the length of one of the former, and there- 

 fore only half its vis viva, yet, in consequence of the equal velo- 

 city of waves of every colour, the impacts or impulses of the violet 

 series will be twice as frequent as those of the red. The same 

 principle applies to any intermediate colour ; and hence it follows 

 that every colour in the spectrum ought to have an equal heat- 

 ing-power. 



Description of the Apparatus employed. 



The optical arrangement T have employed for carrying the 

 foregoing suggestions into practice is represented by fig. 2, and 

 in a horizontal section by fig. 3. 



. A ray of sunlight reflected by a Silbermann's heliostat comes 

 into a dark room through a slit a, 1 milUm. wide. It then 

 passes through a prism b. On the front face of this prism is a 

 black paper screen, cc, having a rectangular opening just suffi- 

 cient to permit the light of the slit to pass. After refraction the 

 dispersed rays fall as a spectrum on a concave metallic mirror, 

 dd, 9 inches in aperture and 11 in focus for parallel rays. I 

 have sometimes used one of speculum-metal, but more frequently 

 one of glass silvered on its front face. In front of this mirror 

 there are therefore three foci. At a distance of eleven inches 

 there is one, e, giving a spectrum-image of the sun. Still 

 further there is a second,/, 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 further off^, at g, is a focal 

 image of the rectangular opening of the black paper [c c) on the 

 front face of the prism. This image, arising from the recombi- 

 nation of all the dispersed rays, is consequently white. These 

 second and third foci are at distances from the mirror depending 

 on the distances of the slit a and the black paper c c respectively. 



With the intention of being certain that the light coming 

 through the slit a is falling properly on the rectangular opening 

 in the prism screen cc, a small looking-glass is placed at j9. 

 The experimenter, sitting near the multiplier m, can then see 

 distinctly the reflected image of that opening. 



At the place where the second focal image with its Fraunhofef 

 lines forms, two screens of white pasteboard {h, i) are arranged. 

 By suitably placing the former of these, h, the more refrangible 

 rays may be intercepted ; and in like manner by the other, 2, the 

 less refrangible. In using these screens, and particularly h. 



