Royal Society. 145 



oscillations in a given time, L may be determined in terms of 

 R and c ; for if the current is reversed n times per second, 

 then 



L= 2^?( 1 + v/I - c2R2 " V )- 



Electrical vibrations due to induction occur under various 

 circumstances. For example, the "false discharge" from a 

 submarine cable ; the oscillatory phenomena described by M. 

 Blaserna and others; and Mr. Edison's " getheric-force " ex- 

 periments. 



XX. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 71.] 



Feb. 10, 1876.— Dr. J. Dalton Hooker, C.B., President, in the Chair. 



'T'HE following papers were read : — 



" On Repulsion resulting from Radiation." — Part III. By 

 William Crookes, E.R.S. &c. 



This paper contains an account of experiments on the action of 

 radiation on bodies the surfaces of which have their radiating and 

 absorbing powers modified by various coatings. The difference 

 between a white and a lampblacked surface in this respect was at 

 first not very decided ; and experiments have been instituted with 

 the object of clearing up some anomalies observed in the actions. 

 Two pith disks, one white and the other black, were suspended on 

 a light arm in a glass bulb by means of a fine silk fibre ; after 

 perfect exhaustion the w T hite and black disks were found to be 

 equally repelled by heat of low intensity, such as from the fingers, 

 warm water, &c. A copper ball was then tried at gradually in- 

 creasiug temperatures. IJp to 250° C. it repelled both equally, 

 above that the black was more repelled than the w T hite, and at a full 

 red heat the repulsion of the black disk was very energetic. A 

 lighted candle acts with more energy than the red-hot copper. 



The presence of even a small quantity of aqueous vapour in 

 the exhausted apparatus almost, if not quite, neutralizes the more 

 energetic action which luminous rays appear to exert on a mackened 

 surface. 



After describing several different modifications and some new 

 forms of apparatus devised to facilitate experiment, the author gives 

 a drawing of an instrument which enables him to get quantitative 

 measurements of the amount of incident light falling on it. It 

 consists of a flat bar of pith, half black and half white, sus- 

 pended horizontally in a bulb by means of a long silk fibre. A 

 small magnet and reflecting-mirror are fastened to the pit ; hand 

 a controlling magnet is fastened outside so that it can slide up 

 and down the tube, and thus increase or diminish sensitiveness. 

 The who]e is completely exhausted, and then enclosed in a box 



Phil. Mag. S. 5. Vol. 2. No. 9. Aug. 1876. L 



