476 Royal Society. 



rate of one revolution in 6*95 seconds, the concave side being 

 attracted. 



These experiments show that the repulsive action of radiation 

 on the convex side is about equal to the attractive action of radiation 

 on the concave side, and that the double speed with which the fly 

 moves when no screen is interposed is the sum of the attractive and 

 repulsive actions. 



No. 1037. A two-disk cup-shaped aluminium radiometer, as 

 above, lampblacked on the concave surfaces. 



In this instrument the action of light is reversed — rotation taking 

 place, the bright convex side being repelled, and the black concave 

 attracted. 



That this attraction is not apparent only, is proved by shading 

 off the sides one after the other. When the light shines only on 

 the bright convex side no movement is produced ; but when it shines 

 on the black concave side, this is attracted, produciug rotation. 



No. 1038. A cup-shaped radiometer similar to the above, but 

 having the convex surfaces black and the concave bright. 



Light shining on this instrument causes it to rotate rapidly, the 

 convex black surface being repelled. No movement is produced on 

 letting the light shine on the bright concave surface ; but good rota- 

 tion is produced when only the black convex surface is illuminated. 



No. 1039. A cup-shaped radiometer like the above, but blacked 

 on both sides. 



With this a candle causes rapid rotation, the convex side being 

 repelled. On shading off the light from the concave side the 

 rotation continues, but much more slowly ; on shading off the convex 

 side the concave is strongly attracted, causing rotation. 



When either of these four radiometers is heated by a hot shade 

 or plunged into hot water, rotation is always produced in the 

 opposite direction to that caused by the light. On removing the 

 source of heat the motion rapidly stops, and then commences in the 

 opposite direction (i. e. as it would rotate under the influence of 

 light), the rotation continuing as long as the fly is cooling. Chilling 

 one of these radiometers with ether has the opposite action to ex- 

 posing it to dark heat. 



The vanes of radiometers have also been formed of metal cones, 

 and of cups and cones of plain mica, roasted mica, pith , : paper, &c. ; 

 and they have been used either plain or blacked on one or both 

 surfaces. These have also been balanced against each other, and 

 against metal plates, cups, and cones. The results are of conside- 

 rable interest, but too complicated to explain without great expen- 

 diture of time and numerous diagrams. The broad facts are con- 

 tained in the above selections from my experiments. 



The action of light on the cup-shaped vanes of a radiometer 

 probably requires more experimental investigation before it can be 

 properly understood. Some of the phenomena may be explained on 

 the assumption that the molecular pressure acts chiefly in a direction 

 normal to the surface of the vanes. A convex surface would there- 

 fore cause greater pressure to be exerted between itself and the 



