[S] A more reliable method of imparting to the surface of the paper a high gloss is to squeegee it down upon a ferrotype plate, the surface of which requires no preparation, and the risk of sticking is nil.—Trans.
[T] For etching extremely delicate transfers a decoction of Aleppo gall with gum is to be recommended. 15 gr. of crushed nut galls are boiled for one hour in half a kilo of water and strained through a cloth. To etch the transfer add a few drops of this solution to a solution of gum of a syrupy consistency; spread over the stone with a soft sponge, and allow to dry.
CHAPTER XIX.
Negatives for Photo-lithography.
AN unconditionally good negative is required for photo-lithography, and those who have not seen one produced for the purpose by an expert should endeavour as a first step to do so, or they will scarcely be likely to devote the attention necessary for producing one of a thoroughly suitable character. Not only must the lines of the copy be represented in the negative by clear glass, and the white ground of the original be represented by opacity, but the utmost sharpness of definition of the finest dots and lines to the very corners of the plate is absolutely necessary to render the negative thoroughly serviceable. To produce such a negative a lens entirely free from distortion, and capable of giving microscopic definition, is, of course, a first necessity.[U]
[U] Lenses of the rectilinear or symmetrical class are best.
The Wet Collodion Process is better adapted than any other for the production of suitable negatives, possessing the necessary density and freedom from fog or veil. The bath must work with the utmost cleanliness, and the collodion should not be used too soon after iodising. The developer works better when not quite fresh, and should contain a sufficiency of acid, and the negative, after fixing, should be subjected to a vigorous intensification.
Professor Husnik thus comments upon negatives for photo-lithography:—