172). ;W. Draper—Distribution of Heat in the Spectrum. 
The proper position of the intercepting screens h, 7, can often 
be verified with precision by looking through blue cobalt glass. 
This glass insulates a definite red, an orange, and a yellow ray 
in the less refrangible regions, and then commencing with the 
green, gives a continuous band to the end of the violet. Its 
red ray begins at the less refrangible end of the spectrum, and 
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 concave mirror giving 
a white image on the face of the pile. ‘ 
The screen h is then to be placed so that, without inten 
any rays coming from the prism to the mirror, it cuts off 
the Fraunhofer spectrum above H?. Be 
e screen 7 is so placed as to cut off all rays less refrangible 
than the sodium line p. More correctly, the screen should be 
a little beyond p. The light on the face of the pile will now 
be greenish-blue. ; 
The screen n is then placed so as to intercept the intromitted 
beam. When the needles of the multiplier come to rest they 
give the working zero, which must be noted. ne 
The intromitting screen n being now removed, the multiplier 
will indicate all the heat of the more refrangible rays, that } 
from a little beyond p up to H?._ The force corrected for the 
working zero is to be noted. ; 
screen 7 is then removed to the line A, so as to give all 
the radiations between the lines A and H?. The light on the 
