236 PRACTICAL PHOTO-MICROGRAPHY 



yellow stain has disappeared. It must not be washed in 

 running water, or the image is again reduced. If any stain 

 remains after several successive washings, a weak solution 

 of ammonium sulphocyanide may be used to remove it. This 

 intensifier, under suitable conditions, is perhaps one of the most 

 useful, as the possible increase of density is very great ; but 

 unfortunately the results are not entirely stable. 



Another method of intensification, and the one most often 

 used, is as follows. The negative is bleached in a saturated solu- 

 tion of bichloride of mercury to which about J c.c. of hydrochloric 

 acid has been added to each 100 c.c. of solution. The negative 

 is allowed to remain in this until it is bleached right through to 

 the back, and is then taken out and thoroughly washed. It 

 may be blackened by immersion either in weak ammonia, 

 10 % solution of sodium sulphite, or any alkaline developer 

 such as pyro-soda or metol-hydrokinone, already mentioned. 

 The blackening by ammonia gives the greatest amount of 

 intensification, but the results are not so stable as with either 

 of the two last mentioned. Intensification with mercury is 

 in some cases very useful, but it results in an increased 

 granularity of the film; in some cases this is so marked 

 that it shows definitely in the print. A more permanent 

 method, and one that gives a finer deposit, is after bleaching 

 with the bichloride of mercury to immerse the negative in the 

 following solution : 



Saturated solution of potassium oxalate, 6 parts ; 

 Saturated solution of ferrous sulphate, 1 part. 



The ferrous sulphate must be added to the ferrous oxalate, 

 otherwise a yellow precipitate is thrown down ; but the latter may 

 be re-dissolved by the addition of a small quantity of the ferrous 

 oxalate solution. The negative is immersed in this until 

 blackened throughout, and is then well washed and dried. 

 If the increase of density is not sufficient, the operation may 

 be repeated again and again; but between each a thorough 

 washing of the plate must be carried out. 



Reducing. In cases where excessive density has been 

 obtained, usually by over-development, the process of reducing 

 may be resorted to. In the class of work under consideration, 

 that known as Howard Farmer's Reducer is perhaps the most 

 useful. It has the particular property of reducing the thin 



