202 THE APPLICATION OF PHOTOGRAPHY TO THE 



But, lately, we have had placed in our hands a medium that 

 is free from this serious defect ; a medium which, while it obtains 

 for us a much more sensitive surface, enables us to give any 

 amount of exposure, without the least inconvenience. Another 

 great advantage we now possess is that microscopic lenses are 

 better corrected, and the luminous and actinic foci are now 

 brought much more into unison than in those lenses made 

 twenty or thirty years ago. 



The subject resolves itself into four sections : — 



The first relates to the preparation of the sensitive surface. 



The second to the mechanical contrivances for placing this 

 sensitive surface in a position to receive the luminous image. 



The third, to the optical means of producing this image ; and 



The fourth, to the chemical processes connected with the 

 development of the latent image. 



It will not be needful to say more than a few words on each of 

 these sections. 



First, as regards the sensitive surface. This is formed of an 

 emulsion of Bromide of Silver, in gelatine, which is spread 

 on the surface of a plate of glass as evenly as possible, and 

 allowed to dry quietly, being, at the same time, carefully protected 

 from dust and light. Owing to the exceeding sensitiveness of 

 the Silver Bromide to light, very great care must be taken to 

 prevent the access of any light except such as has no actinic pro- 

 perties. The process of preparing these plates is so difficult, and 

 the causes of failure so numerous, that it will seldom be worth the 

 while of any person to make them for himself, especially as they 

 are now sold of standard quality, and at very moderate prices, 

 by several firms. It may appear invidious to name any special 

 maker, but those which I have used, and intend to use in illus- 

 tration of this paper, are made by Swans, of Newcastle. 



Second, the camera to be used for Photo-micrography differs 

 only from the ordinary camera in being capable of much greater 

 extension in length. In reality we are going through a directly 

 reverse operation to that of the ordinary photograph. There we 

 have a large object and a greatly reduced image ; here we have a 

 minute object and a greatly enlarged picture. The form of 



