Mr. W. Crookes on the Otheoscope. 71 



7. Radiometer. — A vertical radiometer, made with eight disks of 

 mica blacked on one side, and the whole suspended on a horizontal 

 axis which works in two glass cups. The motion of the radiometer 

 is assisted on each side by driving vanes of aluminium blacked on 

 one side. 



8. Radiometer. — A vertical turbine radiometer, the oval vanes of 

 roasted mica blacked on one side. 



9. Radiometer. — A spiral radiometer of roasted mica blacked on 

 the upper side. 



10. Radiometer of large size, showing great sensitiveness. 



11. Radiometer. — A two-disk radiometer, the fly carrying roasted 

 mica disks blacked on one side ; in front of each black surface is 

 fixed a large disk of thin clear mica. The molecular disturbance 

 set up on the black surface, and streaming from it, is reflected in 

 the opposite direction by the clear plate of mica, causing the fly 

 to move abnormally, *. e. the black surface towards the light. 



12. Radiometer. — A two-disk radiometer, the fly carrying roasted 

 mica disks blacked on one side, similar to No. 11, but with a large 

 clear disk on each side. The molecular disturbance, prevented 

 from being reflected backwards by the second clear disk, is thus 

 caused to expand itself in a vertical plane, the result being a total 

 loss of sensitiveness. 



13. Radiometer. — A two-disk, cup-shaped, aluminium radiometer, 

 facing opposite ways ; both sides bright. Exposed to a standard 

 candle 3*5 inches off, the fly rotates continuously at the rate of 

 one revolution in 3'37 seconds. A screen placed in front, so as to 

 let the light shine only on the convex surface, produces repulsion 

 of the latter, causing continuous rotation at the rate of one revolu- 

 tion in 7*5 seconds. When the convex side is screened off, so as 

 to let the light shine only on the concave, continuous rotation is 

 produced at the rate of one revolution in 6*95 seconds, the concave 

 side being apparently attracted. These experiments show that the 

 repulsive action o£ 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. 



14. Radiometer. — A two-disk, cup-shaped, aluminium radiometer, 

 lamp blacked on the concave surfaces. In this instrument the usual 

 action of light is reversed, rotation taking place, the bright convex 

 side being repelled, and the black concave attracted. When the 

 light shines only on the bright convex side, no movement is pro- 

 duced ; but when it shines on the black concave side, this is at- 

 tracted, producing rotation. 



15. Radiometer. — A cup-shaped radiometer similar to the above, 

 bat having the convex surfaces black and the concave bright. Light 

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

 black being repelled. No movement is produced on letting the 

 light shine on the bright concave surface, but good rotation is 

 produced when only the black convex surface is illuminated. 



16. Radiometer. — A multiple-disk, cup-shaped, turbine radio- 



