346 The Mechanical Action of Light. (July, 
centre of the tube on to the plates gf’ causes g to be at- 
tracted and /’ to be repelled, as shown by the light reflected 
from the mirrors, cc’. The atmospheric pressure in the 
apparatus is equal to about 40 millims. of mercury. 
In a torsion apparatus similar to the one shown in Figs. 4 
and 5 I have submitted variously coloured discs 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 sur- 
faces. ‘The result was to show a decided difference between 
the action of light and of radiant heat. At the highest ex- 
haustions dark 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 lumi- 
nous 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 shown at 
Fic. 6. 
Fig. 4, one disc 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 ation will produce rapid rotation 
in one direction, which keeps up until stopped by the torsion 
of the suspending fibre. 
Taking advantage of this fact I have constru¢ted an in- 
strument 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, suspended on a hard steel 
