231 



posit in the film, therefore no actinic light was admissible until the plate was 

 fixed. 



The developer was the ferrous oxalate, made by a saturated solution of 

 protosulphate of iron being added to a saturated solution of neutral oxalate 

 of potash, in the proportion of one part of the iron to three parts of the 

 oxalate ; these were best when freshly mixed. If the plate had been rightly 

 exposed the image began to make its appearance in about forty seconds, and 

 grew under the action of the developer until it was full of detail. When the 

 image showed faintly through the film on looking at the back of the plate, 

 the development should be stopped, and the plate washed by a good rinsing 

 in rain or distilled water. It was then placed in the fixing bath, which con- 

 sisted of a solution of four ounces of hyposulphite of soda dissolved in a 

 pint of water, that extracted all the silver which had not been acted upon by 

 the light. The plate must then be thoroughly washed to rid it of the hypo- 

 sulphite of soda ; this was done by placing it in running water under a tap 

 for half an hour. If the film contained a trace of hyposulphite it caused the 

 image to fade out after a time. 



Some plates had a tendency to frill, that was to say, the edges of the gela- 

 tine film, while wet, had a tendency to separate from the glass and to curl up. 

 To prevent this the plate might be soaked for about five minutes in a saturated 

 solution of alum, and then again well washed and stood up on a sheet of 

 blotting paper to dry spontaneously. 



Mr. White remarked that he had given these practical details before com- 

 mencing the demonstration, to enable the members present to follow the 

 practical part with a clear understanding of what he was doing, and why he 

 did it, and at the same time the box and its contents had been getting warm, 

 a not unimportant item in the proceedings, for if operations were begun 

 before that took place, the glasses would become dewed with the moisture 

 condensed s on them, the brass wouldbe in a state of expansion from the heat, 

 the image would be wanting in definition, and the plate assuredly spoiled. 



The room having been darkened, Mr. White proceeded to carry out the 

 various processes he had described, the object photographed being a blow- 

 fly's tongue. He then printed a positive picture of the same object on paper 

 by the light of an ordinary paraffin lamp. 



The demonstration was followed by a discussion, and the apparatus was 

 afterwards exhibited in the Library, and further explanations given on various 

 points. 



The President moved a vote of thanks to [Mr. White, which was carried 

 unanimously. 



The following objects were exhibited in the Library : — 



Outer and Inner Epidermal Layer of Apple ... Mr. F. W. Andrew. 

 Marine Alga. Callhthamnion byssoideuml Mr T H B ffl am 



in fruit ) 



Branched Hairs, Flower of Anigozanthus\ m F ft k 11 



flavidus ) 



Tingis hystricellus, from Ceylon Mr. F. Enock. 



Sections of various Echinus spines Mr. H. Hensoldt. 



Diatoms. Stictodiscus Bury anus Mr. H. Morland. 



