488 



SCIENCE. 



[N. S. Vol. XI. No. 274. 



means a latent pictui-e is formed, which is 

 rendered visible in precisely the same way 

 as the light pictures are. 



The substances which produce on a pho- 

 tographic plate these results so strongly re- 

 sembling those produced by light, are, some 

 of them, metallic, while others are of vege- 

 table origin. At first it seemed very re- 

 markable that bodies so different in charac- 

 ter should act in the same way on the 

 photographic plate. The following metals^ 

 magnesium, cadmium, zinc, nickel, alumin- 

 ium, 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 verj' slight ex- 

 tent. The action in general is much slower 

 than that of light, but under favorable con- 

 ditions a picture 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 

 state it is without the power of acting on a 

 photographic plate, but scratch it or scrape 

 it, and it is easy to prove that the bright 

 metal is active. I would saj' that all the 

 pictures which I have to show you, by means 

 of the lantern, are produced by the direct 

 action of the metal, 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 °C. 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 photo- 

 graphic plate, and a very strong and sharp 

 picture is the result. The time required to 

 produce these zinc pictures varies very much 

 with the temperature. At ordinary temper- 

 ature 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 photographic plate would 

 be damaged. Contact between the zinc and 

 photographic plate is not necessary, as the 

 action readily takes place through consider- 

 able 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 pic- 

 tures, 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 vai'ies, 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 pic- 

 tures produced 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 pic- 

 ture is produced. 



The second class of bodies which act 

 in the same way on a photographic plate 

 are organic substances, and belong essen- 

 tially to the groups of bodies known as ter- 

 penes. In trying to stop the action of 

 metallic zinc, which I thought at the time 



