PHOTOGRAPHIC PRINTING. 127 



light, but it refuses to absorb water. Advantage is taken of this 

 fact to prepare surfaces from which pictures can be printed in 

 printers' inks in an ordinary type or lithographic press. If a glass 

 plate be coated with a mixture of gelatine and potassium dichro- 

 mate (with an addition of either gum resin or chrome alum to 

 give the film hardness), and be then exposed to the action of light 

 that passes through a negative, the different portions of the film 

 of gelatine will have become absorbent of water in the exact 

 proportion to the intensity of the light acting through the negative. 

 If the plate be plunged into cold water for a time, and after the 

 excess of water has been Sponged off, a soft roller, coated with 

 greasy ink, be passed over the gelatine, the full depth of ink will 

 adhere to those portions of the image which have been strongly 

 acted upon by light, and the different shades of the picture will 

 be formed by the varying proportions of greasy ink that the film 

 of gelatine is capable of holding, a power that is altogether de- 

 pendent on the intensity of the light that has acted on it, and the 

 consequent varying absorption of water. When a. picture is thus 

 formed in greasy ink, the gelatine film (which may be supported 

 on the original glass plate or on a metal plate) may be placed 

 in the printing-press, paper laid on it, and an impression taken. 

 The surface may then be damped and rolled up again, and more 

 impressions pulled. The Albertype, Heliotype, Pantotype, and 

 the Autotype mechanical processes are all of them modifications 

 of the above. 



Photo-lithography is equally dependent on the reducing action 

 of light on dichromated gelatine. In some processes the gela- 

 tine with which paper is coated, and which has not been acted 

 upon by light, is washed away; the whole surface of the film 

 having been coated with a layer of greasy ink subsequent to its 

 exposure beneath a negative picture of a plan or map. In such 

 examples the lines are formed of altered gelatine holding on its 

 surface a layer of fine ink. The transfer, as this finished print is 

 termed, is placed on a lithographic stone, pulled through the 

 lithographic press, and impressions taken in the usual way. 



