110 M. E. Pringsheim on the Radiometer. 



That this effect indeed proceeds only from the glass case is 

 known from the fact that even a plane glass plate of 8 millim. 

 thickness, placed between the source of light and the apparatus, 

 is sufficient to prevent any deflection. 



As the deflection of the translucent mica vane when irradia- 

 tion takes place from the front arises from the excess of the 

 amount of heat absorbed by the fore half of the glass case 

 over that absorbed in the hinder half, it will be possible to 

 reverse the motion by so arranging the glass case that its 

 hinder half absorbs more heat than the fore half. This can 

 be done by blackening the hinder half with lampblack. But 

 when this was done and the apparatus irradiated by a bright 

 Bunsen burner, there appeared at first not a reversal, but only 

 a weakening of the deflection, and only by inserting glass in 

 the path of the rays could a reversal be obtained. Conse- 

 quently with full irradiation the heating of the clean half of 

 the glass was stronger than that of the blackened half, and it 

 was only when the rays most absorbable by glass were removed 

 before reaching the sphere that the absorption in the lamp- 

 black gained the preponderance. In this experiment the 

 resting-position of the spot of light was : — 



Without With full J^M* ^ Sm i? e( J 



illumination. illumination. throu ^ h 15 f l lim ' thlck " 



ness 01 glass. 



500 489 554 



Deflection ... 11 54 



It was different when sunlight was employed, which had the 

 greatest effect without any intervening glass, and produced a 

 deflection of 50 millim.— a fresh proof that extremely little of 

 solar radiation is absorbed by glass. 



All the above experiments prove that the heated glass side 

 exerts upon a movable surface suspended in front of it a repel- 

 lent force which is independent of the substance of the surface 

 — or, differently expressed, that a pressure emanates from the 

 heated glass side which increases with the temperature of the 

 glass. 



In numerous similar experiments it has by previous observers 

 been ascertained that any warmed surface tends to repel a light 

 body suspended over against it in a space of rarefied air. 



Therefore, having now proved that this repulsion is a direct 

 one, and not generated by absorption of the heat radiated by 

 the warmed body, we can express the above proposition in the 

 following amplified form : — 



From a warm surface, in a space containing rarefied air, 



