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NICHOLS AND HULL. — PRESSURE DUE TO RADIATION. 571 



t 

 stray light. Just beyond 

 the shutter and attached 

 to the diaphragm rf G was 

 a 45° glass plate which 

 reflected a part of the beam 

 to the lens L & , by means 

 of which an image of d 3 

 was projected upon one 

 arm of a bolometer at R. 

 The glass lens L 3 focused 

 a sharp image of the aper- 

 ture d 3 in the plane of the 

 vanes of the torsion bal- 

 ance B l under the bell-jar. 

 The bell-jar was provided 

 with three plate glass win- 

 dows W 1} W 2 , W 3 . 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 3 

 was arranged to move hor- 

 izontally between the stops 

 S 3 and £4. These were so 

 adjusted that when the lens 

 was against S 3 the sharp 

 image of the aperture d 3 

 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 

 T 2 , mounted on the car- 

 riage of a dividing engine. 

 This was used to observe 

 and measure the position nj 



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