114 Dr. J. W. Draper on the Distribution of 



The operations now 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 concave mirror giving 

 a white image on the face of the pile. 



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

 any rays coming from the prism to the mirror, it cuts off all in 

 the Fraunhofer spectrum above H^. 



The screen, i is so placed as to cut off all rays less refrangible 

 than the sodium-line D. More correctly this screen should be 

 a little beyond D. The light on the face of the pile will now be 

 greenish blue. 



The screen n is then so placed as to intercept the intromitted 

 beam. When the needles of the multiplier come to rest they 

 give the working zero, which must be noted. 



The intromitting screen n being now removed, the multiplier 

 will indicate all the heat of the more refrangible rays — that is, 

 from a little beyond D up to H^. The force, corrected for the 

 working zero, is to be noted. 



The screen i is then removed to the line A, so as to give all 

 the radiations between the lines A and H^. The light on the 

 face of the pile is white, and the multiplier gives the w"hole heat 

 of the visible spectrum. By subtracting the foregoing measure 

 from this, we have the heat of the less-refrangible region — that 

 is, from A to the centre of the spectrum. 



As a matter of curiosity the experimenter may now, if he 

 pleases, remove the screens h, i ; the light on the face of the pile 

 will still be white, and the multiplier will give the force of the 

 entire radiations, except so far as they are disturbed by the ther- 

 mochrose of the m.edia. These measures, as not bearing upon 

 the problem under consideration, I do not give in the following 

 Tables. 



Instead of advancing the screen i from the less toward the 

 more refrangible regions, I have very frequently moved h from 

 the more refrangible regions tow^ard the less. When it is 

 brought down froDi H^ to the centre of the spectrum, the light 

 on the face of the pile is of an intense orange-red ; it might 

 perhaps be called a bromine-red. I need not give further de- 

 tails of this mode of experimentation, as I did not find that its 

 results differed in any important degree from those obtained as 

 just described. 



The variation in different experiments may generally be traced 

 to errors in placing the screen i with exactness on the centre of 

 the spectrum and on the line A. 



For the sake of more convenient comparison I have reduced 

 all the different sets of experiments to the standard of 100 for 

 the whole visible spectrum. 



