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S. P. Langley—Selective Absorption of Solar Energy. 189 
h 
the eye-piece are set upon the D line the circle should indicate 
a deviation of 47° 41’ 15’. A bright and pure image of the 
spectrum about 6™" wide and 640™™ long between the A and 
a Thomson reflecting astatic galvanometer of about 20 ohms 
resistance, constructed especially for the purpose by Elliot 
Bros. of London. It is placed upon a pier entirely disconnected 
from the building. The scale is cylindrical, with divisions 1™ 
apart on a transparent surface, and is placed 1 meter from the 
vanometer requires no reduction for our present purpose. A 
resistance forming the Wheatstone’s bridge, and other 
electrical adjuncts of the bolometer are on the ri ft 
