110 Dr. William James Russell [May 5, 



WEEKLY EVENING MEETING, 



Friday, May 5, 1899. 



Sir Frederick Bramwell, Bart., D.C.L. LL.D. F.B.S., Honorary 

 Secretary and Vice-President, in the Chair. 



William James Btjssell, Esq., Ph.D. V.P.R.S. M.B.I. 



Pictures Produced on Photographic Plates in the Dark. 



I think I may fairly assume that every one in this theatre has had 

 their photograph taken, and consequently must have some idea of the 

 nature of the process employed. I have, therefore, only to add, with 

 regard to what is not visible in the process of taking the picture, that 

 the photographic plate is a piece of glass or such like body, coated on 

 one side by an adhesive paste which is acted on by light, and acted on 

 in a very remarkable manner. No visible change is produced, and the 

 picture might remain latent for years, but place this acted-on plate in a 

 solution, of, say pyrogallol, and the picture appears. The subsequent 

 treatment of the plate with sodium hyposulphite is for another purpose, 

 simply to prevent the continuance of the action when the plate is 

 brought into the light. Now, what I purpose demonstrating to you to- 

 night is that there are other ways of producing pictures on photo- 

 graphic plates than by acting on them by light, and that by these other 

 means a latent picture is formed, which is rendered visible in precisely 

 the same way as the light pictures are. 



The substances which produce on a photographic plate these results 

 so strongly resembling those produced by light, are, some of them, 

 metallic, while others are of vegetable origin. At first it seemed very 

 remarkable that bodies so different in character should act in the same 

 way on the photographic plate. The following metals — magnesium, 

 cadmium, zinc, nickel, aluminium, lead, bismuth, tin, cobalt, antimony 

 — are all capable of acting on a photographic plate. Magnesium most 

 strongly, antimony but feebly, and other metals can also act in the 

 same way, but only to a very slight extent. The action in general is 

 much slower than that of light, but under favourable conditions a 

 jiicture may be produced in two or three seconds. 



Zinc is nearly as active as magnesium or cadmium, and is the most 

 convenient metal to experiment with. In its ordinary dull ^tate it is 

 without the power of acting on a jmotographic plate, but scratch 

 it or scrape it, and it is easy to prove that the bright metal is active. 

 I would say that all the pictures which I have to show you, by 

 means of the lantern, are produced by the direct action of the metal, 



