THE MECHANICAL ACTION OF LIGHT. 



691 



light reflected from the mirrors, e c'. The atmospheric pressure in 

 the apparatus is equal to about forty millimetres of mercury. 



In a torsion-apparatus similar to the one shown in Figs. 4 and 5 

 I have submitted variously-colored disks to the action of the different 

 rays of the spectrum. The . most striking results, as yet, have been 

 obtained when the different rays of the spectrum were thrown on 

 white and on black surfaces. The result was to show a decided differ- 

 ence between the actions of light and of radiant heat. At the high- 

 est exhaustions dai-k heat from boiling water acts almost equally on 

 white pith and on pith coated with lamp-black, repelling either with 

 about the same force. The action of the luminous rays, however, is 

 different. These repel the black surface more energetically than they 

 do the white surface, and, consequently, if in such an apparatus as is 



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tSr- 



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e 

 Fig. 6. 



shown at Fig. 4, one disk of pith is white and the other is black, an 

 exposure of both of them to light of the same intensity will cause 

 the torsion-thread to twist round, owing to the difference of repulsion 

 exerted on the black and the white surface. If, in the bulb-apparatus 

 shown in Fig. 3, the halves of the pith-bar are alternately white and 

 lamp-blacked, this differential action will produce rapid rotation in 

 one direction, which keeps up until stopped by the torsion of the sus- 

 pending fibre. 



Taking advantage of this fact I have constructed an instrument 

 which I have called the Radiometer, shown in section and plan at 

 Figs. 7 and 8. It consists of four arms, of some light material, sus- 

 pended on a hard steel point resting in a jewel-cup, so that the arms 

 are able to revolve horizontally upon the centre pivot, in the same 

 manner as the arms of Dr. Robinson's anemometer revolve. To the 

 extremity of each arm is fastened a thin disk of pith, white on one 

 side and lamp-blacked on the other, the black surfaces of all the disks 

 facing the same way. The whole is inclosed in a thin glass globe, 



