294 



A similar method has been recommended by Greenall (3). 

 The writer uses m.uch more dilute solutions, and gains control 

 and variation of tone unobtainable with the concentrations 

 recommended by Greenall. In both methods it will be seen 

 that control of tone is gained, other things being equal, by 

 regulating the amount of silver haloid formed, this in turn 

 determining the amount of silver sulphide which is subse- 

 quently formed. 



The Copper Method. 



Here the silver image is treated with copper sulphate, 

 in the presence of potassium ferricyanide, together with — in 

 the case considered — neutral potassium citrate. 



The change in colour is supposedly due to the formation 

 of copper and silver ferrocyanides. 



The Troubjle. 

 In all processes for toning bromide photographic prints 

 it is considered essential, in order to obtain satisfactory 

 results, to : — 



1. Expose the print to just that stage where prolonged 



development will not reduce more silver than is 

 essential to the production of a normal fully- 

 developed print. 



2. Use fresh developer. 



3. ''Fix" very thoroughly with fresh "hypo." 



4. Wash fully i-i cider to remove the soluble silver 



sodium salts formed, together with any free 

 "hypo." (This is not required when using the 

 chlorine "bleacher.") 



5. Dry prints before toning. 



6. Use in the sulphide process fresh sodium sulphide 



solution. It is preferable to use solutions made up 

 from a concentrated stock, prepared by pouring 

 boiling water on the sodium sulphide, or by bring- 

 ing the concentrated solution to a boil. A con- 

 centrated solution so prepared is very stable. 



Now, it sometimes happens that when all these precau- 

 tions have been taken, unsatisfactory tones occur, especially in 

 the copper process. Here, when only toning to the purple- 

 blacks and purples, the tone is apparently quite good. Pro- 

 ceeding to the warmer tones, the contrast values decrease at 

 a faster rate than the colour contrast change would warrant. 

 The tone ceases to be homogeneous. The shadows, where the 

 silver deposit was rich, tones through to the warmer tones, 

 whilst the lighter half-tones and high-lights become many 

 tones colder. 



