472 Royal Society:— 



bring the arms into contact, theblack surface of the mica then having 

 a clear plate of mica in front of it. On bringing a lighted candle 

 near the instrument, and allowing it to shine through the clear 

 plate on the blackened mica, the clear plate is at once driven away 

 till the arm sets at right angles to the other. 



Two currents of force, acting in opposite directions, can exist 

 in the same bulb. I have prepared a double radiometer in which 

 two flies are pivoted one over the other, and having their blackened 

 sides turned in opposite directions. On bringing a lighted candle 

 near, the flies rapidly rotate in opposite directions. 



Experiment shows that the force can be reflected from a plane 

 surface in such a manner as to change its direction. If an ordinary 

 radiometer is exposed to light the black surface is repelled, owing 

 to the excess of pressure acting between it and the glass. If, 

 however, a plate of mica were to arrest this force and reflect it 

 back again, the motion should be reversed. Experiment shows that 

 this is the case. A two-disk radiometer was made, having flat 

 opaque mica disks blacked on one side. In front of the black surface 

 of the mica, about a millimetre off: , is fixed a large disk of thin clear 

 mica. On bringing a candle near, the molecular pressure streaming 

 from the black surface is caught by the clear plate and thrown 

 back again, causing pressure behind instead of in front ; and the 

 result is rapid rotation in the negative direction, the black side now 

 moving towards the light. 



To still further test this view of the action, I made another radio- 

 meter, similar to the above, but having a clear mica disk on each 

 side of the ordinary mica vane. This prevents the reflection of 

 the pressure backwards, and causes it to expend itself in a vertical 

 plane, the result being an almost total loss of sensitiveness. 



The above actions can be explained on the " evaporation and 

 condensation " theory, as well as by that of molecular movement ; 

 and I therefore devised the following test to decide between these 

 two theories. A radiometer has its four disks cut out of very 

 clear and thin plates of mica, and these are mounted in a some- 

 what large bulb. At the side of the bulb, in a vertical plane, 

 a plate of mica, blacked on one side, is fastened in such a position 

 that each clear vane in rotating shall pass it, 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 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. This could not happen on the evaporation and condensation 

 theory, as this requires that the light should shine intermittently on 

 the black surface in order to keep up continuous movement. 



By cutting a thin plate of aluminium into the form of a spiral, 

 then drawing it out corkscrew fashion, blacking the upper surface 

 and suspending it on a point, a spiral radiometer is made, which 

 rotates like a screw on exposure to light. Here also the black 

 surface need never be in darkness, the pressure acting continuously 



