M. B. Pringsheim on the Radiometer. 109 



In order to put this dependence to the proof, the transparent 

 mica vane employed in the previous experiments was blackened 

 on one side with soot and again inserted in the apparatus. 

 The source of light was placed at the side of the apparatus (as 

 in the experiments described in pp. 106 and 107), in the plane 

 of the vane, so that the part of the glass case in front of the 

 vane was as strongly illuminated as the half situated behind it. 



If the action depends on the substance of the vane, with 

 this arrangement the irradiation of one half of the glass sphere, 

 while the other half and the vane are shaded, ought to produce 

 a quantitatively different deflection from that produced by the 

 irradiation of the other half. On the other hand, if the action 

 consists of a direct repulsion, it is evident that it will remain 

 unaltered whether it strikes a soot or a mica surface, and then 

 the deflections produced by both halves of the glass case must 

 be of the same magnitude. The experiment confirmed the 

 latter supposition. At the same time it should not be for- 

 gotten that all other influences, especially those of neighbouring 

 objects and the walls of the apartment, must be carefully kept 

 off from the apparatus, as in this form it is very sensitive, and 

 the arrangement of the experiment makes it impossible to 

 surround it with a protecting case. 



As with lateral irradiation of the entire glass case direct 

 heating of the soot is evidently unavoidable, in the following 

 Table the deflections are reckoned from the resting-position 

 when the entire apparatus is completely shaded. 



The experiments, conducted with the requisite precautions, 

 gave the following result: — 



Resting-position of the Spot of Light on Shading 



The blackened 

 side of the glass. 



Bright gas-flame =.. 545 





The entire 

 glass case. 



500 





The mica-side 

 of the glass. 



455 



j 



Deflection . . . 

 Bright gas-flame ... 576 



45 

 1 



500 



J v. 



4 



7 



5 

 1 



425 



j 



Deflection . . . 

 Dull gas-flame ... 555 



V" 



76 

 1 



500 



5 

 1 



445 



Deflection ... 

 Dull gas-flame ... 570 



5 

 7 



r 



5 



500 



J v. 



V 



55 

 1 



430 





f" 









7 









