1877.] 



Mr. W. Crookes on the Otheoscope. 



177 



ment as the one here sketched out is capable of an almost endless variety 

 of forms ; and as it is essentially different in its construction and mode 

 of action to the radiometer, I propose to identify it by a distinctive 

 name, and call it the Otheoscope (tbdew, I propel). 



The glass bulb is an essential portion of the machinery of the radio- 

 meter, without which the fly would not move ; but in the otheoscope the 

 glass vessel simply acts as a preserver of the requsite amount of rarefac- 

 tion. Carry a radiometer to a point in space where the atmospheric pressure 

 is equal to, say, one millimetre of mercury, and remove the glass bulb ; 

 the fly will not move, however strong the incident radiation. But place 

 the otheoscope in the same conditions, and it will move as well without 

 the case as with it. 



In the preliminary note already referred to*, I described a piece of appa- 

 ratus by which I was able to measure the thickness of the layer of mole- 

 cular pressure generated when radiation impinged on a blackened surface 

 at any degree of exhaustion. At the ordinary density of the atmosphere 

 the existence of this molecular disturbance was detected several millimetres 

 off, and its intensity increased largely as the generatiug surface and mov- 

 able plate were brought closer together. It would be possible, therefore, 

 to construct an otheoscope in which no rarefaction or containing vessel 

 was necessary, but in which motion would take place in air at the normal 

 density f. Such a heat-engine would probably work very well in sun- 

 light. 



Aided by the mechanical dexterity of my assistant, Mr. C. H. Griming- 

 ham, I have constructed several varieties of otheoscope. These will be 

 exhibited at the Soiree of the Royal Society on Wednesday next, as 

 illustrations of the very beautiful manner in which, at this stage of my 

 investigations, theory and experiment proceed hand in hand, alternately 

 assisting each other, and enlarging our knowledge of these laws of mole- 

 cular movement which constitute a key to the relations of force and 

 matter. 



The following is a list of the otheoscopes I have already made, together 

 with some new experimental radiometers, which will be exhibited for the 

 first time on Wednesday : — 



1. Otheoscope. — A four-armed fly, carrying four vanes of thin clear mica, 

 is mounted like a radiometer in an exhausted glass bulb. At one side 

 of the bulb a plate of mica blacked one side is fastened in a vertical plane, 

 in such a position that each clear vane in rotating shall pass the plate, 

 leaving a space between of about a millimetre. If a candle is brought 

 near, and by means of a shade the light is allowed to fall only on the 

 clear vanes, no motion is produced ; but if the light shines on the black 



* Proc. Eoyal Soc. Nov. 16, 1876, p. 310. 



t Since writing this I have constructed such an instrument. The movement takes 

 place in the wav I had anticipated. — W. C, April 26th, 1877. 



N 2 



