Heat in the Spectrum, 111 



care must be taken that no rays passing jfrom tlie prism to the 

 mirror are obstructed — a remark that applies especially to the 

 invisible rays of less refrangibility than the red. For this reason 

 the mirror dd must be placed at such an obliquity to its inci- 

 dent rays as to throv/ the focal images sufficiently on one side. 

 Yet this obliquity must not be greater than is actually necessary 

 for that purpose, or the purity of the second spectrum with its 

 Fraunhofer lines will be interfered with. At the place of the 

 third focus, arising from the reunion of the dispersed rays, is the 

 thermopile g, connected by its wires k k with the multiplier m. 



Whenever any of the visible rays of the Eraunhofer spectrum 

 are intercepted by advancing either of the screens h or i, the 

 image on the face of the pile ceases to be white. It becomes of 

 a superb tint, answering to the combination of the non-inter- 

 cepted rays. A slip of white paper placed for a moment in front 

 of the pile v>all satisfy the experimenter how magnificent these 

 colours are. It is evident, therefore, that by this arrangement 

 the pile will enable us to measure the heat of any particular 

 ray or of any selected combination of rays. The screens can be 

 arranged so as to reach any designated Fraunhofer line. 



The pile I have used is of the common square form; a linear 

 pile would not answer. The focal image on the pile is of very 

 much greater width than the slit a, on account of the obliquity 

 of the front face of the prism. 



By removing the screen h and placing the screen i 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 

 general diffusion of light by the substance of the prism. Under 

 these circumstances the image on the pile will be white, and the 

 multipher will give a deflection representing the heat of the 

 visual and the ultra-violet regions. If, then, the screen be ad- 

 vanced still further until it has intercepted all the less-refran- 

 gible regions up to the sodium line D, or a little beyond (that is, 

 to the optical centre 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, 

 however, may be eliminated by properly using the other screen, h. 

 Besides 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 recognized 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 a 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. 



