ELEMENTARY PHOTO-MICROGRAPHY. 83 



makeshift. Mercuric chloride is a deadly poison 

 which should be kept under lock and key, and not 

 be brought into contact with the fingers. After 

 the negative has been freed from hypo it may be 

 placed in the following solution until the film 

 becomes white : 



Mercuric bichloride . . J ounce. 



Hydrochloric acid . . f dram. 



Water .. . .. 10 ounces. 



It is then thoroughly washed and redeveloped 

 with a weak solution of ammonia, or any ordinary 

 developer, which will darken the film. It is again 

 well washed and dried. 



Over-dense negatives may be reduced in opacity 

 by immersion in equal parts of fresh fixing bath 

 (hypo) and water and a few drops of potassium 

 ferricyanide solution. If the reduction be too slow, 

 more ferricyanide solution may be added. The 

 plate should be lifted out repeatedly for examina- 

 tion, as the action is generally rapid. It is then 

 washed and dried. 



