THE PEESSUKE DUE TO RADIATION, 



121 





mercury cup and serving as a rough manometer for measuring the 

 larger gas pressures emplo3'ed during the observations. The low 

 pressures were measured on the ISIacLeod gauge in the usual way. 

 A semicircular magnet M, fitted to 

 the vertical curvature of the bell jar, 

 was used to direct the suspended 

 magnet 111., and thus to control the 

 zero position of the torsion balance. 

 By turning M through 1S0\ the op- 

 posite faces of the vanes C and D 

 could be presented to the light. 



THE AKRANGEMENT OF APPAKATUS. 



A horizontal section of the appa- 

 ratus through the axis of the light 

 beam is shown in fig. 2. The white- 

 hot end of the horizontal carbon S^ 

 of an A. T. Thompson 90- arc lamp, 

 fed by alternating current, served as 

 a source. The arc played against 

 the end of the horizontal carbon 

 from the vertical carbon, which was 3 

 screened from the lenses L^ and L, >- 

 })y an asbestos diaphragm d^. A 

 lens, not shown,, projected an en- 

 larged image of the arc and carbons 

 on an adjacent wall, so that the posi- 

 tion of the carbons and the condition 

 of the arc could be seen at all times 

 ]>v lioth observers. 



The cone of ravs passing through 

 the small diaphragm J, fell upon the 

 glass condensing lenses L^, Lg. ^ 



At cZg a diaphragm, 11.25 mm. in 

 diameter, was interposed, which per- 

 mitted onlv the central portion of 

 the cone of rays to pass. Just be- 

 yond d.j^ the beam passed to a shutter r- 

 at S.3. This shutter was worked by liij[2 

 a magnetic escapement, operated by 

 the second's contact of a standard 



clock. The observer at T^ might choose the second for opening or 

 closing the shutter, but the shutter's motion alwa3's took place at 

 the time of the second's contact in the clock. Any exposure was 



