1899.] on Pictures Produced on Photographic Plates in the Dark. 141 



or whatever the active body may be, on the photographic plate, and 

 that they have not been intensified or touched up in any way. This 

 first slide is the picture given by a piece of ordinary zinc which has 

 been rubbed with some coarse sand-paper, and you see the picture 

 of every scratch. Here is a piece of dull zinc on which some circles 

 have been turned. It was exposed to the photographic plate for four 

 hours at a temperature of 55° 0. In the other cases, which are on a 

 larger scale, a zinc stencil was polished and laid upon a photographic 

 plate, and you see where the zinc was in contact with the plate much 

 action has occurred. In another case a bright zinc plate was used, 

 and a Japanese stencil interposed between it and the photographic 

 plate, and a very strong and sharp picture is the result. The time 

 required to produce these zinc pictures varies very much with the tem- 

 perature. At ordinary temperature the exposure would have to be for 

 about two days, but if the temperature was, say, 55° C, then half to 

 three-quarters of an hour might be sufficient. Temperatures higher 

 than this cannot be used except for very short times, as the photo- 

 graphic plate would be damaged. Contact between the zinc and 

 photographic plate is not necessary, as the action readily takes place 

 through considerable distances. Obviously, however, as you increase 

 the distance between object and plate, so you decrease the sharpness 

 of the picture, as is shown by the following pictures, which were taken 

 respectively at a distance of 1 mm. and 3 mm. from the scratched zinc 

 surface. The appearance of the surfaces of different metals varies, 

 and the following slides show the surface of a plate of bismuth, a plate 

 of lead, and one of aluminium. On the next slide are the pictures pro- 

 duced by similar pieces of pure nickel and cobalt, and it clearly shows 

 how much more active in this way nickel is than cobalt. Many alloys, 

 such as pewter, fusible metal, brass, etc., are active bodies, and in 

 the case of brass the amount of action which occurs is determined by 

 the amount of zinc present. Thus you will see that a brass with 30 

 per cent, of zinc produces hardly any action on the photographic plate, 

 but when 50 per cent, of zinc is present there is a fairly dark picture, 

 and when as much as 70 per cent, is present a still darker picture is 

 produced. 



The second class of bodies which act in the same way on a photo- 

 graphic plate are organic substances, and belong essentially to the 

 groups of bodies known as terpenes. In trying to stop the action of 

 metallic zinc, which I thought at the time might arise from vapour 

 given off by the metal, copal varnish was used, but in place of stop- 

 ping the action it was found to increase it, and this increase of 

 activity was traced to the turpentine contained in the varnish. In ex- 

 perimenting with liquids it is convenient to use small shallow circular 

 glass vessels such as are made for bacteriological experiments, the 

 plate resting on the top of the vessel, and the amount of liquid in the 

 vessel determining the distance through which the action shall take 

 place. The following slide, produced in this way, shows how dark a 

 picture ordinary turpentine produces. All the terpenes are active 



