Prof. Minchin's Experiments in Photoelectricity. 207 



Experiments upon which I have been engaged since the 

 above paper has been in type fully confirm these expectations. 

 Preliminary measurements show that the force with which 

 the glass suspended in water tends to move toward the weaker 

 parts of the field is several times greater than that with which 

 it tends to move toward the stronger parts of the field in air, 

 and that, other things being the same, this force is propor- 

 tional to the square of the electric force. 



A conductor, on the contrary, tends to move toward the 

 stronger parts of the field for all media, with a force many 

 times greater in water than in air or oil. Increasing the 

 conductivity of the water does not appreciably alter the 

 force. 



A new piece of apparatus is being constructed, and when 

 finished accurate measurements will be made of the force on 

 the suspended body in these several cases. The preliminary 

 results are of interest, however, as confirming the conclusions 

 of the foregoing paper that water and other conducting 

 liquids have a true specific inductive capacity. — E. B. R. 



XXV. Experiments in Photoelectricity. By George M. 

 Minchin, M.A., Professor of Mathematics in the Royal 

 Indian Engineering College, Coopers Hill*. 



NEAR the beginning of the year 1877 I commenced a long- 

 series of investigations in Photoelectricity, and I was 

 not then aware that M. Becquerel had previously worked at 

 this subject and had obtained some results which I also ob- 

 tained independently. In the spring of 1880 I made a verbal 

 communication to this Society, illustrated by experiments, on 

 the generation of electric currents by the action of light on 

 silver plates which were coated with collodion and gelatine 

 emulsions of bromide, chloride, iodide, and other salts of silver, 

 as well as with eosine, fluorescine, and various aniline dyes. 

 A short summary of these results appears in the Report of the 

 meeting of the British Association that year at Swansea. 

 The chief object which I had then in view in prosecuting such 

 experiments was the solution of the problem of producing a 

 photographic image of an object at a distance by means of an 

 instrument which is still imaginary and which I proposed to 

 call a telephotograph. 



As these experiments have never been published in ade- 

 quate detail, I propose to begin this paper with an account of 

 them. 



* Communicated by the Physical Society : read January 16, 1891. 



