398 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII. 



less refrangible end of the spectrum, and ends near C ; 

 it includes the fixed lines A, B, C. Its orange ray lies 

 wholly between C and D, including neither of those lines. 

 Its yellow ray begins near 5894, and ends about 5581 ; 

 the line D is therefore near its point of beginning. The 

 remaining continuous band begins about 5425 ; it there- 

 fore includes the lines E, b, F, G, H. I have found this 

 glass of much use in determining how far the screen i 

 has been pushed. It is convenient to select a light kind 

 of it, and by looking through one, two, or three pieces, 

 the depth of color can be regulated at pleasure. 



The optical train which has acted on the sunbeam un- 

 der examination is, therefore, (1) the sun's atmosphere ; 

 (2) the earth's atmosphere ; (3) the heliostat mirror of 

 speculum metal ; (4) the prism ; (5) the concave mirror 

 of silvered glass ; (6) the blackened face of the thermo- 

 pile. 



Results obtained by the Apparatus. 



We are now ready to examine the results which this 

 optical apparatus yields, it having been of course pre- 

 viously ascertained that the reflecting band of the con- 

 cave mirror, d d, is sufficient to receive all the radiations 

 coming from the prism, and that none are escaping past 

 its edges. 



The operations required are as follows : 



The heliostat is to be set, and its reflected ray brought 

 into the proper position. The optical train is adjusted, 

 the prism being at its minimum deviation, and the con- 

 cave mirror giving a white image on the face of the 

 pile. 



The screen h is then to be placed so that, without in- 

 tercepting any rays coming from the prism to the mirror, 

 it cuts off all the Fraunhofer spectrum above H 2 . 



The screen i is so placed as to cut off all rays less re- 

 frangible than the sodium line D. More correctly, the 



