NICHOLS AND HULL. — PRESSURE DUE TO RADIATION. 571 



stray light. Just beyond 

 the shutter and attached 

 to the diaphragm d^ was 

 a 45° glass plate which 

 reflected a part of the beam 

 to the lens Zg, by means 

 of which an image of d^ 

 was projected upon one 

 arm of a bolometer at R. 

 The glass lens Zg focused 

 a sharp image of the aper- 

 ture rfg in the plane of the 

 vanes of the torsion bal- 

 ance Bi under the bell-jar. 

 The bell-jar was provided 

 with three plate glass win- 

 dows JTi, W^, W^. The 

 first two gave a circular 

 opening 42 mm. in diam- 

 eter, and through the third, 

 deflections of the balance 

 were read by a telescope 

 and scale. The lens L^ 

 was arranged to move hor- 

 izontally between the stops 

 S^ and aS'4. These were so 

 adjusted that when the lens 

 was against S^ the sharp 

 image of the aperture d^ 

 fell centrally upon one 

 vane ; and when against S^ 

 the image fell centrally 

 upon the other. This ad- 

 justment, which was a very 

 important one, was made 

 by the aid of a telescope 

 7*2, mounted on the car- 

 riage of a dividing engine. 

 This was used to observe 

 and measure the position 



s 



a 

 w 



B 

 CO 



.Si. 



■1= 



-'^ 



i" 



Ki 



