522 Mr M°Glelland, On the figures produced [May 16, 



graphs obtained by him of the spectrum of the corona, and also 

 the two series of photographs of the spectrum of the sun's limb 

 at the beginning and end of totality. 



Monday, May 16, 1898. 



Mr F. Darwin, President, in the Chair. 



At a Meeting of the Society held at the Cavendish Laboratory, 

 B. Cookson, Trinity College, was elected an Associate. 



The following Communications were made to the Society: 



(1) On the figures produced on photographic plates by electric 

 discharges. By J. A. M c Clelland, M.A., Trinity College. 



Several experimenters have lately drawn attention to the 

 figures produced on photographic plates on development, after 

 an electric discharge has taken place on to or near the surface 

 of the plates. The figures are very interesting and well denned, 

 and differ in a marked degree according as the discharge is 

 positive or negative. 



The manner of the production of these figures seems to be 

 imperfectly understood, and it has not been shown whether they 

 are due to a direct chemical action of the discharge on the plate, 

 or whether they are simply produced by the light of the discharge. 

 If the figures are produced by a direct chemical action on the 

 plate it would point to a difference in the manner in which the 

 positive and negative discharges spread over the plate ; if produced 

 by the action of light on the plate the figures would be explained 

 by the different forms of the discharge in the air close to the plate. 



In this paper the effect of the gaseous medium surrounding 

 the discharging point on the form of the figures is investigated, 

 and also the effect of interposing thin plates of different material 

 between the film and the discharge. In this way we vary the 

 form of the visible discharge in the gas close to the plate and 

 can compare it with the figure produced, and we test the trans- 

 parency of various substances to the action. 



It will appear from the figures given that the effect on the 

 films is produced principally by the light of the discharge. 



The discharge passes through the air quite close to the film 

 and the sharpness of the lines on the figures is explained by the 

 nearness of the source of light to the plate, only the part of the 

 plate just beneath the lines of discharge being affected. 



On this explanation the difference in the positive and negative 

 figures corresponds to the difference in the discharges in air ; and 

 the photographic action brings out these differences very clearly. 





