124 THE PRESSURE DUE TO RADIATION. 



exposures were measured. In the static observations each vane of the 

 bahmce was exposed in turn to the beam from the lamp, the exposiu'es 

 lasting" until tiie turning- points of the swing's showed that stationary 

 conditions had l)een reached. The moment of pressure of radiation 

 and gas action combined would thus be equal to the product of the 

 static deflection and the constant of the balance. The torsion S3^stem 

 was then turned through ISO- by rotating the outside magnet, and 

 similar observations were made on the reverse side of the vanes. All 

 turning points of the swinging balance in these observations were 

 recorded. From the data thus obtained the resultant of the combined 

 radiation and gas forces could be determined for the time of ever}^ 

 turning point. Every value was divided by the deflection at standard 

 sensitiveness of the g-alvanometer G„ read at the same time, and was 

 thus reduced to standard lamp. Results thus obtained, together with 

 the ballistic measurements, show^ed the direction and extent of the gas 

 action as well as its variation with length of exposure. 



The reasons for reversing the suspension follow: The ])eam from 

 the lamp, before reaching the balance, passed through three thick 

 glass lenses and two glass plates. All wave lengths destructiveh" 

 absorbed by the glass were thus sifted out of the beam by the time 

 it reached the balance, vanes. The silver coatings on the vanes 

 absor])ed, therefore, more than the glass. The radiation pressure was 

 always away from the source, irrespective of the way the vanes were 

 turned, while the gas action would be exerted mainly on the silvered 

 sides of the vanes. 



At the close of the pressure and energy measurements, when the 

 reflecting power of the silver faces of the vanes was compared with 

 that of the glass-silver faces, the reflection from the silver faces was 

 found very nmch higher than that for the glass faces backed ])y silver. 

 This result was the more surprising because the absorption of the 

 unsilvered vanes was found b}' measurement to be negligil)l3" small." 

 This unexpected difi'erence in reflecting power of the two faces of the 

 mirrors prevented the elimination of the gas action, by the method 

 descril)ed, from being as complete as had been hoped for. But by 

 choosing a gas pressure where the gas action after long exposure is 

 small, the whole gas efl'ect during the time of a l)allistic exposure may 

 be so reduced as to be of little consequence in any case. 



By exposing each of the vanes in turn and b}' reversing the suspen- 

 sion and averaging results, nearly all errors due to lack of symmetry 

 in the l)alance or in the position of the light images with reference to 

 the rotation axis, or errors due to lack of uniformit}" in the distribu- 

 tion of intensity in different parts of the image, could l)e eliminated. 



« Lord Rayleigh records a similar difference between the reflection from air-silver , 

 and glass-silver surfaces. Scientific paj^ers, Cambridge, 2, 538-539, 1900. 



