light reflected from the mirrors, c 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 difference between the actions of light and of radiant heat. At the highest exhaustions 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 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
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 suspending 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, suspended 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,