122 



THE PRESSURE DUE TO RADIATIOl^. 



thus of some wliolo number of seconds' duration. The opening in 

 the shutter was such as to kH through, at the time of exposure, all 

 of the direct beam which passed through r/.,, Ijut to shut out stray 

 light. Just be^'ond the shutter and attached to the diaphragm d^ was 

 a 45^ glass plate, which reflected a part of the beam to the lens L^, by 

 means of which an image of d.^ was projected upon one arm of a bolo- 

 meter at R. The glass lens L., focused a sharp image of the aperture 

 d.^ in the plane of the vanes of the torsion balance Bj, imder the bell 

 jar. The l)ell jar was provided with three plate-glass windows W^, 

 Wg, W3. The first two gave a circular opening- 42 mm, in diameter, 

 and through the third deflections of the balance were read by a 

 telescope and scale. The lens L3 was arranged to move horizontally 

 between the stops S3 and 8^. 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 



To giilv. Gi 



To galv. Go 4^ 



Fig. 



other. This adjustment, which was a very important one, was made 

 b}' the aid of a telescope T.,, mounted on the carriage of a dividing 

 engine. This was used to observe and measure the position of the 

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

 lens L3 was against the stops. For the latter measurements the vanes 

 could ])e moved out of the wa}' by turning the suspension through 

 90'^ b}^ the control magnet M (fig. 1). 



To make sure that the balance as used was entirel,y free from any 

 magnetic moment or disturl)ance, 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 

 vertical 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 posi- 

 tions of the maernet. 



