572 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



of the rotation axis, as well as the positions of the images of d^, when the 

 lens X3 was against the stops. For the latter measurements, the vanes 

 could be moved out of the way by turning the suspension through 90° 

 by the control magnet M (Fig. 2). 



To make sure that the balance as used was entirely free from any 

 magnetic moment or disturbance, the small magnet m,, was clamped in 

 one position to maintain a constant zero, and the period of the balance 

 was accurately measured with the axis of the large magnet M in the ver- 

 tical plane of the vanes and again when the axis was at right angles 

 to the plane of the vanes. Several series of this sort failed to show a 

 difference of 0.1 second in the period of the balance for the two positions 



of the magnet. 



Togalv. G^ 



To galv. Gf: 



Figure 4. 



The bolometer at R (Fig. 3) was of sheet platinum 0.001 mm. thick, 

 rolled in silver. The strip was cut out in the form shown in Fig. 4, and 

 mounted on a thin sheet of slate *S'. Two windows had been cut in the 

 slate behind the strips at ABCD where the silver had been removed 

 leaving the thin platinum. The platinum surfaces were blackened by 

 Kurlbaum's process. The image from Z5 (Fig. 3), fell at D. The sil- 

 ver ends between A and C were connected with E and F respectively. 

 On the heavy wire EF a sliding contact c served to balance the bridge, 

 all four arms of which are shown in the figure. 



Methods of Observation. 



The observations leading to the results given later were of three differ- 

 ent kinds : (1) The calibration of the torsion balance ; (2) the measure- 



