174 S. P. Langley on the Selective Absorption of Solar Energy. 



of the mirror M exactly bisects the angle between the direc- 

 tion of the arm A' and the central line of the track; so that 



Fig. 4. 



>' 



^ 



M 



a ray falling on the centre of the mirror from the centre of the 

 instrument at P, after reflection falls upon the bolometer-strips. 

 C, C f are counterpoises to 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 

 in line with the long tube. The eyepiece being placed in the 

 y's at B, the image of the distant slit is brought upon the 

 central wire, when the reading of the divided circle should be 

 0° 00 / 00", indicating a deviation of zero. The arm is then 

 moved to one side as in the figure, until the mirror intercepts 



