1899.] 07i Pictures Produced on Photographic Plates in the Dark. 148 



it, as is seen in this picture of a piece of perforated zinc, a picture 

 which was produced at ordinary temperatures. Gold-beaters' skin, 

 albumen, collodion, gutta-percha, are also bodies which are transparent 

 to the action of the zinc and the other active bodies. On the other 

 hand, many bodies do not allow the transmission of the action through 

 them ; for instance, paraffin does not, and among common substances 

 writing-ink does not, as is easily shown by placing ordinary paper 

 with writing on it between the active body and the photographic 

 plate. The active body may conveniently be either a plate of zinc 

 or a card painted with copal varnish and allowed to dry, or a dish of 

 drying oil. The picture of an ordinarily directed envelope shows 

 this opacity of ink well. It is a property long retained by the ink, 

 as this picture of the direction of a letter, written in 1801, shows; 

 also this letter of Dr. Priestley's, dated 1795 ; and here is also some 

 very faded writing of 1810, which still gives a very good and clear 

 picture. Even if the writing be on parchment, the action passes 

 through the parchment, but not through the ink, and hence a picture 

 is formed. 



With bodies which are porous, such as most papers, for instance, 

 the action passes gradually through the interstices, and impresses the 

 plate with a picture of the general structure of the intervening sub- 

 stance. For instance, the following pictures show the structure and 

 the water-mark of certain old and modern writing-papers. Some 

 modern writing-papers are, however, quite opaque; but usually paper 

 allows the action to take place through it, and combining this fact 

 with the fact of strong activity of the printing-ink, the apparently 

 confused appearance produced on obtaining a picture from paper with 

 printing on both sides is accounted for, as the printing on the side 

 away from the photographic plate, as well as that next to it, prints 

 through the paper, and is, of course, reversed. 



I hope I have now given you a clear idea how a picture can be 

 produced on a photographic plate in the dark, and the general character 

 and appearance of such pictures. I now pass on to the important 

 question of how they are produced. Moser suggested fifty years ago 

 that there was " dark light," which gave rise to pictures on polished 

 metallic plates, and lately it was suggested that pictures were produced 

 by vapour given off by the metals themselves ; the explanation, how- 

 ever, which I have to offer you is, I think, simpler than either of these 

 views, for I believe that the action on the photographic plate is due to 

 the formation of a well-known chemical compound, hydrogen peroxide, 

 which undergoing decomposition acts upon the plate and is the im- 

 mediate cause of the pictures formed. The complicated changes 

 which take place on the sensitive plate I have nothing to say about 

 on the present occasion, but I desire to convince you, that this 

 body, hydrogen peroxide, is the direct cause of these pictures pro- 

 duced in the dark. Indirect proof has to be resorted to. Water 

 cannot be entirely excluded, for an absolutely dry photographic plate 

 would probably be perfectly inactive, and as long as water is present 



