188 8S. P. Langley—Selective Absorption of Solar Energy. 
table over the vertical axis of the massive instrument we have 
now, of New York, from the writer’s design. 
Two long arms, A, A’, turn independently about: the above 
mentioned axis, the angle between them being measured by a 
10’. One of 
eylinder, and serves to examine optically the place which will 
be occupied by the bolometer strips when the bolometer cylin- 
der is in the y’s. The optical axis of the mirror, M, exactly 
bisects the angle between the direction of the arm, A’, and the 
central line of the track, so that a ray falling on the center of 
the mirror from the center of the instrument at P, after reflec- 
tion falls upon the bolometer strips. ©, C’ are counterpoises t0 
offset the weight of the arms A, A’. 
To adjust the apparatus for observation, the screws at D are 
‘ loosened, the prism removed, and the arm A’ brought around 
sere tal oe 
