198 METHODS OF MICROSCOPICAL RESEARCH. 



during or after development and in very hot 

 weather all plates have these tendencies they must 

 be soaked for about five minutes in the alum solu- 

 tion, both before and after fixing, to harden the 

 films. In hot weather one should not wait for the 

 commencement of frilling or blistering before apply- 

 ing the remedy, but take it for granted that it will 

 occur, and pass every plate through the alum. It 

 can do no harm in any case. 



Plates developed with " pyro " acquire a pale 

 brown or yellow colour, which, if strongly marked, 

 it may be advisable to remove by a few minutes' 

 immersion in the hydrochloric acid solution. Over- 

 exposed or under-developed plates can be intensified 

 as follows : The mercuric chloride solution is first 

 applied and kept in motion until the negative 

 becomes a very pale grey, almost white. If the 

 operation be conducted in a black dish the negative 

 will appear as a most beautiful positive when the 

 whitening is complete. The plate should then be 

 thoroughly washed for at least half-an-hour in 

 several changes of water. The weak ammonia 

 solution should then be applied, and in a few 

 seconds will blacken the image. It will then want 

 a thorough washing in several changes of water, 

 prolonged through about six hours, after which it 

 may be stood in a rack to dry. 



When dry, the negatives must be varnished with 

 photographic varnish flowed over them while hot. 

 This will protect the films from mechanical injury 

 aud from silver stains in printing. When negatives 

 have been intensified, it is especially necessary to 

 varnish them as soon as possible, as they are liable 

 to turn white by exposure to the air. 



