THE MECHANICAL ACTION OF LIGHT. 265 



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 coat- 

 ed with lampblack and with other substances, another torsion appa- 

 ratus 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 compari- 

 sons 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 <ff 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 sus- 

 pended 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 con- 

 struction 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, J5, and small 

 magnet, (7, 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 exhaust- 

 ed and then inclosed in a box lined with black velvet, with apertures 

 for th J ays of light to pass in and out. A ray of light from a lamp, 

 F, reflected from the mirror, , 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 



