26 



time either before or after coating, or even exposing, the 

 plates. 



Development is almost universally effected in England 

 by p37rogallol and ammonia in the same way as with collodion 

 plates. But abroad a solution of ferrous oxalate is used, 

 which works more slowly, and has been thought to allow of 

 more latitude in the exposure. 



Thus the radical essentials of the formation of silver 

 bromide in the emulsion, its washing to eliminate the soluble 

 nitrate also formed, and the development of the picture, are 

 practically unchanged, and it only remains to speculate, for I 

 fear I cannot do more, on what may be the cause of the 

 alteration in the effects produced by the new process. 



The action of light on a sensitive film is no doubt 

 mechanical. The interesting contrivance known as the 

 radiometer, I think, demonstrates this, and it is besides 

 difficult to conceive how light per se can set up chemical 

 reaction. That the impression on the film is not caused by 

 the heat rays is also certain. It therefore results that the 

 bromide in the film must either be more accessible to the 

 blow which its exposure to light entails, or that the medium 

 which contains it must permit the effect of this blow to be 

 more deeply, for I must not say widely, transmitted by means 

 of the developer. Unfortunately gelatine is so complex and 

 inconstant a body, that it is extremely hard to trace any 

 definite change in it, caused by its treatment in emulsifying. 

 But it is to the use of gelatine rather than to any modifica- 

 tion of the sensitive salt that we should, I think, look for an 

 explanation of the increased rapidity and other phenomena 

 which I have mentioned. I must, however, offer this sug- 

 gestion with the greatest diffidence, as I make it without 

 authority, and I must apologize, by anticipation, if it should 

 prove to be misleading. I may, in conclusion, tender my 

 thanks to the Society for the honour which has been done 

 me in allowing me to open what I hope may be a valuable 

 and not an unrepeated discussion on photography. 



