178 



Mr. W. Crookes on the Otheoscope. [Apr. 26, 



plate, the fly instantly rotates as if a wind were issuing from this surface, 

 and keeps on moving as long as the light is near. 



2. Otheoscope. — A four-armed fly carries roasted mica vanes, and is 

 mounted in an exhausted glass bulb like a radiometer. Fixed to the 

 side of the bulb are three plates of clear mica, equidistant from each 

 other in a vertical plane, but oblique to the axis. A candle brought near 

 the fixed plates generates molecular pressure, which, falling obliquely on 

 the fly, causes it to rotate. 



3. Otheoscope. — A large horizontal disk revolving by the molecular 

 disturbance on the surface of inclined metallic vanes, which are blacked 

 on both sides in order to absorb the maximum amount of radiation. 



4. Otheoscope. — Inclined aluminium vanes driven by the molecular 

 disturbance from the fixed black mica disk below, blowing (so to speak) 

 through them. 



5. Otheoscope. — A large horizontal coloured disk of roasted mica, driven 

 by inclined aluminium vanes placed underneath it. 



6. Otheoscope. — A bright aluminium disk cut in segments, and each 

 segment turned at an angle, driven by a similar one below of lampblacked 

 silver. 



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, i. 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 con- 

 tinuous rotation at the rate of one revolution in 7*5 seconds. When the 



