M. E. Pringsheim on the Radiometer, 



107 



Re sting -position of the Spot of Light : On the Illumination 



Of the entire sphere. 



Of the fore half. 

 Bright gas-flame ... 468 



Deflection ... 

 Dull gas-flame . . . 447 



500 



_J V_ 



half. 



531 



; 



32 



31 



-v— 



53 



500 



552 



52 



Deflection ., 



II. With lateral irradiation from the side situated furthest 

 from the mica plate : — 



Resting-position of the Spot of Light : On the Illumination 

 Of the fore half. I Of the entire sphere. 

 Bright gas-flame ... 496 500 



Deflection 



~v— 



4 



half. 



504 



; 



Dull gas-flame 



493 



500 



_J ^_ 



507 



Deflection ... 7 7 



These experiments therefore show that in fact the warmed 

 glass side exerts a repellent force upon the vane ; and at the 

 same time it follows, from the difference of the effects when 

 the irradiation proceeded from the side furthest from and from 

 that nearest to the mica plate, that the parts of the glass, 

 situated nearest to the vane almost alone exert the action. 

 This remark will be corroborated by the following experiment. 

 The apparatus was illuminated, as in the first-cited experi- 

 ments, by means of a bright Bunsen burner ; and then the 

 greater part of the light-cone was so far cut off that only the 

 outermost part of the glass, nearest the mica vein, was ir- 

 radiated, while the vane itself was in shadow. The following 

 was the resting-position of the spot of light : — 



With partial illu- 

 mination. 

 471. 



With full illumination. 



465 



Without illumination. 



500 



Deflection 35 



29 



Almost the whole of the effect therefore comes from the 

 part of the glass side immediately opposite to the mica vane. 



After the above experiments with lateral illumination it 

 cannot any longer be doubtful that the deflection is reallv 

 owing to the heating of the glass side ; but from this it does 

 not yet appear whether this effect is a direct repulsion brought 

 about by the air particles present between the two surfaces. 



12 



