278 Mr. W. Crookes on [Feb. 10, 



standard candle 48 inches off was balanced by a gas-flame 113 inches off. 

 The lights were therefore in the proportion of 48 2 to 113 2 , or as 1 to 5|. 

 The gas-burner was therefore equal to 5 J candles. 



By interposing screens of water or plates of alum, and so practically cut- 

 ting off all the dark heat, the actual luminosity is measured. In addition to 

 this, by interposing coloured glasses or solutions, any desired colours can be 

 measured either against the total radiation from a candle, its luminous 

 rays, or any desired colour. One coloured ray can be balanced against 

 another coloured ray, by having differently coloured screens on either 

 side. 



The variations in the luminosity of a " standard " candle will cease to 

 be of importance. Any candle may be taken ; and if it be placed at such 

 a distance from the apparatus that it will give a uniform deflection, say 

 of 100 divisions, the standard can be reproduced at any subsequent time ; 

 and the burning of the cand]e may be tested during the photometric 

 experiments by taking the deflection it causes from time to time, and 

 altering its distance, if needed, to keep the deflection at 100 divisions. 



If the pith bar in this instrument be blacked on alternate halves, an 

 impetus given by a ray of light always acts in the same direction of 

 movement. A candle causes it to spin round very rapidly until the sus- 

 pending fibre is twisted up, and the rotation is stopped by the accumu- 

 lated torsion. 



By arranging the apparatus so that the black and white surfaces are 

 suspended on a pivot instead of by a silk fibre, the interfering action of 

 torsion is removed, and the instrument will rotate continuously under 

 the influence of radiation. To this instrument the author has given the 

 name of the " Radiometer," or " Light-Mill." It consists of four arms 

 of very fine glass, supported in the centre by a needle-point, and having 

 at the extremities thin disks of pith lampblacked on one side, the black 

 surfaces all facing the same way. The needle stands in a glass cup, 

 and the arms and disks are delicately balanced so as to revolve with the 

 slightest impetus. 



In the ' Proceedings of the Royal Society' for 1875 (vol. xxiii. p. 373), 

 the author gave a brief account of some of the earlier experiments 

 with these instruments. In the present paper he enters very fully into 

 the various phenomena presented by them, and gives Tables showing the 

 number of revolutions made by the radiometer when exposed to a con- 

 stant source of light removed different distances from the instrument. 

 The law is that the rapidity of revolution is inversely as the square of 

 the distance between the light and the instrument. 



"When exposed to different numbers of candles at the same distance 

 off, the number of revolutions in a given time are in proportion to the 

 number of candles, two candles giving twice the rapidity of one candle, 

 and three giving three times, &c. 



The position of the light in the horizontal plane of the instrument is 



