The second question—namely, "What influence has the color of the surface on the action?" has also been solved by this apparatus.
In order to obtain comparative results between disks of pith coated with lampblack and with other substances, another torsion apparatus was constructed, in which six disks in vacuo could be exposed one after the other to a standard light. One disk always being lamp-blacked pith, the other disks could be changed so as to get comparisons of action. Calling the action of radiation from a candle on the lampblacked disk 100, the following are the proportions obtained:
| Lampblacked pith | 100 | |
| Iodide of palladium | 87 | .3 |
| Precipitated silver | 56 | |
| Amorphous phosphorus | 40 | |
| Sulphate of baryta | 37 | |
| Milk of sulphur | 31 | |
| Red oxide of iron | 28 | |
| Scarlet iodide of mercury and copper | 22 | |
| Lampblacked silver | 18 | |
| White pith | 18 | |
| Carbonate of lead | 13 | |
| Rock-salt | 6 | .5 |
| Glass | 6 | .5 |
This table gives important information on many points: one more especially—the action of radiation. on lampblacked pith is five and a half times what it is on plain pith. A bar like those used in. my first experiment, having one-half black and one-half white, exposed to a broad beam of radiation, will be pushed with five and a half times more strength on the black than on the white half, and if freely suspended will set at an angle greater or less according to the intensity of the radiation falling on it.
This suggests the employment of such a bar as a photometer, and I have accordingly made an instrument on this principle; its construction is shown in the diagram (Fig. 6). It consists of a flat bar of pith, A, half black and half white, suspended horizontally in a bulb by means of a long silk fibre. A reflecting mirror, B, and small magnet, C, are fastened to the pith, and a controlling magnet, D, is fastened outside so that it can slide up and down the tube, and thus increase or diminish sensitiveness. The whole is completely exhausted and then inclosed in a box lined with black velvet, with apertures for the days of light to pass in and out. A ray of light from a lamp, F, reflected from the mirror, B, to a graduated scale, G, shows the movements of the pith-bar.
The instrument fitted up for a photometric experiment is in front of me on the table. A beam from the electric light falls on the little mirror, and is thence reflected back to the screen, where it forms a spot of light, the displacement of which to the right or the left shows the movement of the pith-bar. One end of the bar is blacked on
