PHOTOGKAPHIC PKOCESSES 237 



parts of a negative without proportionately reducing the 

 denser parts. The result is that in negatives in which great 

 contrast is required, a better result may often be obtained by 

 very full development and by then applying the reducer, the 

 effect being that the dense parts remain practically unaltered, 

 while the thin parts may be completely dissolved out. The 

 solution is made by taking some fresh fixing-solution and 

 adding to it from ten to twenty drops per ounce of a saturated 

 solution of ferricyanide of potassium. It is sufficiently near 

 to estimate the necessary strength by the colour of the resulting 

 solution, as it should be of a pale-yellow colour. It is rapid in 

 action, and should therefore be used rather weak. 



In cases where reduction of the dense parts rather than of 

 the thin parts is required, the following solution should be 

 used : 



Ammonium persulphate . . 25 to 50 grammes. 

 Water to 1000 c.c. 



A fresh solution must be made up at the time of use, pre- 

 ferably in distilled water, and a drop or two of sulphuric acid 

 added to each 100 c.c. The negative must be thoroughly washed 

 before the reducer is applied, and rather before the reducing 

 action is complete must be immersed in a 5 % solution of 

 sodium sulphite. The latter is necessary to ensure the stoppage 

 of the action at the right moment, otherwise the process 

 continues after removal from the persulphate solution. 



If very considerable reduction has been effected, it is 

 advisable to put the plate into a fresh fixing-bath before 

 washing, and finally drying. 



These two reducers will probably be found sufficient for 

 any contingency that may arise, and it is important to realise 

 the great difference which exists between them in their mode 

 of action : the one acts in a manner that increases contrast, 

 and so is useful where black-and-white results only are aimed at ; 

 the other tends to reduce contrast, and may be used in cases 

 where there are wide tone-differences in the negative which 

 renders it difficult to print out detail in the densest parts. 



Printing Processes. A satisfactory negative having been 

 obtained, the next process is to obtain a print on paper from 

 it. To the ordinary photographer there are a large number of 

 processes available; but in photo-micrographic work, where 



