222 PRACTICAL MICROSCOPY. 



and then, if the solution which is in the dish be poured back 

 to the cup a perfect admixture will be the result without 

 the necessity of stirring. With correct exposure full print- 

 ing density may be easily attained with the alkaline pyro. 

 alone. No plate is supplied that will not bear 100 drops 

 of stock solution A* without fog if the correct exposure 

 has been given. One thickness of deep ruby glass is unsafe 

 for the dark room ; two, at least, are necessary. Should any 

 discoloration of the film appear after the negative has been 

 fixed and washed, it may be cleared away with a weak 

 solution of perchloride of iron and water (about the 

 colour of pale sherry), the plate afterwards to be washed 

 thoroughly. 



Ferrous Oxalate Development. Where it is not expressly 

 stated to the contrary, Swan's plates may be developed 

 either with ammoniacal pyrogallic acid or with ferrous 

 oxalate ; but it occasionally happens that one and not the 

 other of these developers must be used. When that is the 

 case, the fact is stated on the label of the packet. 



Ferrous oxalate developing solution is best made as 

 follows : 



SOLUTION A. 



Neutral oxalate of potash 6J oz. 



Water 25 oz. 



SOLUTION B. 



Sulphate of iron 2^ oz. 



Water 7^ oz. 



Sulphuric acid I to 2 drops 



The two solutions A and B are kept in stock, and when 

 required for development three volumes of A are mixed 

 with one volume of B. 



It is better to mix and use what is immediately required 

 for the development of a single plate, and not to use the 

 same solution for developing several plates. The two stock 



