Radiometer is usually a glass vessel containing highly-rarified air. Light vanes are carefully balanced on an axis about which they are free to rotate. If two equal discs are balanced on an arm, and a light be allowed to shine on them, nothing will happen; but if alternate sides be coated with lampblack, rotation will commence in a direction such that the blackened side moves backwards as soon us a candle or other light is brought near. This is due to the fact that the blackened face gets hotter than the mean temperature, and the difference of pressure on the two sides causes motion. Crookes found that a rotation oould also be obtained by using unblackoned vanes if the shape wore altered. He therefore balanced four vanes on two arms, and bent them at the ends; heat then fell upon the convex side of one vane and the concave side of another and rotation occurred. If, further, the vanes be plane and of the same appearance, but of different materials, e.g. one of ohromic oxide and one of copper tungstate, rotation will occur. It has also been shown that the effect varies with the light used. The causo of the motion has been discussed by Osborne Reynolds and by Clerk Maxwell.