1899.] on Pictures Produced on Photographic Plates in the Dark. 145 



was used in the following slides. It is also very easy to obtain good 

 pictures with the peroxide alone of the structure of paper, etc. ; see, 

 for instance, this one of a five-pound note and these of lace. Again* 

 the strict similarity between tbe action of the peroxide and that of 

 the metals and organic bodies is further shown by the fact that its 

 action passes through the same media as their action does ; and hete 

 are good pictures formed by the action of the peroxide after passing 

 through a sheet of these substances. How this singular transmission 

 can be explained, I have treated of elsewhere, and time does not 

 allow of my discussing the matter to-night. 



There are many ways in which the bright, active zinc surface can 

 be modified. Draw your finger across it, press your thumb upon itj 

 and you stop its activity, as is shown by the picture it will give; 

 Lay a printed paper on the zinc, and let the contact continue for 

 three-quarters of an hour, at a temperature of 55°, then bring the 

 zinc in contact with a sensitive plate, a picture of the printing is 

 formed, but allow the contact between the Zinc and printing to 

 continue for eighteen hours at the same temperature, and the picture 

 then given by the zinc is the reverse of the former one. Where the 

 iak has been is now less active than the rest of the plate. Here are 

 slides which show these positive and negative pictures^ Another 

 way of modifying the zinc surface is interesting. You have seen 

 that the ordinary zinc surface which has been exposed to air and 

 moisture is quite inactive, but if a bright piece of zinc be immersed 

 in water for about twelve hours, the surface is acted on ; oxide of 

 zinc is formed, showing generally a curious pattern. Now, if the 

 plate be dried, it will be found that this oxide is strongly active, and 

 gives a good picture of the markings on the zinc. The oxide evi- 

 dently holds, feebly combined or entangled in it, a considerable 

 quantity of the hydrogen peroxide, and it requires long drying or 

 heating to a higher temperature to get rid of it. Also, if a zinc 

 plate be attacked by the hydrogen peroxide, the attacked parts be- 

 come more active than the bright metal. Thus place a stencil on a 

 piece of bright zinc, and expose the plate to the action of an active 

 plaster of Paris slab, or to active blotting-paper for a short titfie$ 

 then, on removing the stencil, the zinc plate will give a very good 

 picture of the stencil. Any inactive body — for instance, a piece of 

 Bristol board or any ordinary soft paper — can be made active by 

 exposing it above a solution of peroxide, or, more slowly, by exposing 

 it to a bright zinc surface. If, for instance, a copper stencil be laid 

 on a piece of Bristol board* and a slab of active plaster of Paris be 

 placed on the stencil for a short time, the Bristol board will even* 

 after it has been removed from the stencil for some time, give a good 

 picture of the stencih Drying oil and other organic bodies may be 

 used in the same way to change the paper. A curious case of this 

 occurred in printing a coloured advertisement cut out of a magazine, for 

 there appeared printing in the picture which was not in the original.- 

 This printing; was ultimately traced to an advertisement on the oppo»- 



Vol. XVI. (No. 93.) " * 



