1878.] to tlie Beproiluction of 2Iaps and Plans. 97 



Or an ink composed of solution of chrome alum may be used and will not 

 require exposure to light. In some cases the part to be corrected may be 

 washed over with a solution of bichromate of potash and allowed to dry, 

 and then the required details may be printed in from another negative. 



The taking out of details is more difficult and requires care. It may 

 be accomplished by washing the part with a strongish solution of caustic 

 potash or cyanide of potassium. Should a plate print dirty, it may be 

 cleaned up and greatly improved by being washed with a weak solution of 

 cyanide of j)otassium, or better, with a solution of citric acid, which not only 

 clears up plates that print dirty, but at the same time facilitates the inking 

 in. A weak solution of ammonia is also said to be useful in this 

 respect. 



The process just described was found to answer better in Calcutta for 

 line-Avork than for half-tones, and for the latter the following formula for 

 the gelatine films appeared preferable : — 



Gelatine, li ounce. 



Glycerine, If dram. 



Albumen, 1 ounce. 



Bichromate of Potash, 40 grains. 



Chrome Alum, 7 grains. 



Water, 12 ounces. 



The j^lates coated with this mixture have to be di-ied in the dark, but 

 in other respects the operations are much the same. 



The processes in which a thick film of gelatine is spread upon a glass 

 plate were found to present in Calcutta many inconveniences in the drying 

 of the films, and the tendency there is in dry weather for the films to peel 

 away from the glass plates and utterly break up and destroy the surface of 

 the latter. There is also the constant risk of breaking the plates in the 

 press. I was therefore led to go back to the old process of Tessie du 

 Mothay and Marechal, in which a thin film of gelatine is supported upon a 

 metal plate, and finally succeeded very well with the following method which 

 I have fully described in the ' Year Booh of Photography' for 1877. 



A flat plate of copper, such as used for engraving, is finely grained on 

 its best side, and having been carefully levelled, is washed with warm water 

 and coated on the grained side, while wet, with a mixture composed of — 



Gelatine (Nelson's opaque), 15 parts. 



Water, 100 „ 



Bichromate of Potash, 4 „ 



Formic acid (when the former are dissolved), 4 „ 



The excess is poured off, so as to leave enough to give a thiu even 

 coating. Half an ounce of gelatine is more than suHicient to cover 450 

 square inches of plate. 



