±6$ Messrs. Abney and Festing on the Transmission of 



But the effect of the induced system at points on the same 

 side of the plane with the given system is equal and opposite 

 to its effect at the corresponding points on the opposite side of 

 the plane . Consequently the induced system has at all points 

 in space on the same side of the plane with the given system 

 the same effect as the optical image of the given system formed 

 on the plane as a mirror. 



24. "We have shown that the proposed law of force leads 

 directly to the correct expression for the energy of any elec- 

 trical system, whether that energy be termed kinetic or poten- 

 tial, and that by the shortest and easiest method. The further 

 development of the theory would generally take the same form 

 as in Maxwell's ' Electricity/ Maxwell's results being obtained 

 generally by the application of Lagrange's equations to this 

 expression for the energy. 



LXI. On the Transmission of Radiation of Low Refrangi- 

 bility through Ebonite. By Capt. Abney, R.E., F.R.S. y 

 and Col. Festing, R.E* 



WHEN Mr. Graham Bell described his interesting expe- 

 riments with the photophone, we were much surprised 

 to learn that an effect was produced when sheets of ebonite of 

 small thickness were interposed between the apparatus and 

 the source of radiation ; and it became a matter of more than 

 curiosity to us to know what was the cause of the phenomenon, 

 since photographic manufacturers were commencing to use 

 ebonite in the construction of the dark slides for the camera. 

 We think we can demonstrate, however, that the ordi- 

 nary explanation of transmission of radiant energy can ac- 

 count for the phenomenon. Dr. Guthrie kindly furnished us 

 with a sheet of ebonite, through which the action of a beam of 

 radiation on a selenium cell was most marked; and we accord- 

 ingly first experimented with that. A photographic spectro- 

 scopic apparatus w T as employed, of the form we have already 

 described in other papers ; and the compound of silver was used 

 which is sensitive to all parts of the spectrum. At first we em- 

 ployed only one prism, and used the sun as a source of illumina- 

 tion; and here it may be parenthetically remarked that on the 

 evening when we made our first experiments the wind was blow- 

 ing from the north-east, and there was a clear sky. Half the slit 

 was covered up, a piece of ebonite placed in front of the other 

 half, and a plate exposed to the action of the spectrum of the 

 radiations (if any) coming through this thin layer of apparently 



* Communicated by the Physical Society, having been read at the 

 Meeting on April 9. 



