The proress of collotype is based upon the peculiar property of gelatine, when sensitised with bichromate of potash and dried at a high temperature, of absorbing water and refusing greasy ink in some places, whilst in others it refuses water but will take the greasy ink. The latter are those parts that have been exposed to light, the former unexposed, and there are degrees between the two. First, a reversed negative is required; this should be thin and soft, such as is suitable for bromide printing. Plate-glass several times larger than the desired picture is finely ground with emery powder and coated with a substratum of albumen and water glass; the plate has to be re-ground each time it is used. Place in a 20 oz. bottle some bits of broken glass and add 4oz. albumen (the white of fresh eggs); 2oz. water glass solution (commercial); and 5oz. water. Shake this violently to a froth, allow to subside and filter through filter-paper. The ground-glass should be well rinsed to remove every particle of emery powder. Grease must be avoided, as this leads to the film tearing from its support, the greatest trouble in collotype. '1 he plate is then coated with the substratum as in varnishing a negative, except that no heat is necessary, and it is dried guarded from dust.

The plate is then again rinsed and dried, and is ready for coating with the sensitive film. Hard gelatine leads to the production of flat prints, and the soft gelatine breaks up after few impressions. Burton advises the use of equal parts of No. 1 and No. 2 (Nelson's) gelatine, and gives the following formula: No. 1 gelatine 1 oz., No. 2 ditto 1 oz., bichromate of potassinm 100 gr., alcohol 1 oz., chrome alum 2 gr., and water 20 oz. The potassinm bichromate is dissolved in 2oz. water, and then ammonia is added till the solution smells. The gelatine is allowed to soften and is then dissolved by heat; the two are mixed and then the chrome alum is added in the form of 40m. of a riper cent, solution. The alcohol is merely added to make the solution flow better, and should be added immediately before use. Finally filter through swans-down calico. The plate is next heated as hot as the hand can hear and held with a holder in the left hand, whilst a pool is poured in the centre. The excess is poured off, and, after a slight rocking to ensure an even film, the plate is placed in the drying oven, the thermometer on the outside of the door of which should indicate a temperature of about 120° F. Drying should take about three hours, not more.

When dry, the plate is ready for printing from the negative. The two films are placed face to face in a stout pressure frame and exposed to a bright light. The negative must have a safe edge fitted as in carbon printing. Printing is judged by an actino-meter, the simplest form of which is a piece of albumen paper exposed behind a thin quarter-plate negative. When printed sufficiently, the plate is laid face down on a sheet of black velvet and the back exposed for a short time to form an insoluble coating near the glass, and to prevent tearing. Development merely consists in washing the plate entirely free from the bichromate, when the lights will be found to have swollen considerably. When drying the plates care must be taken not to opeu the door, or drying marks may be caused. All the operations up to the coating with the bichromate solution may take place in ordinary light, and even after coating the plates are comparatively insensitive until dry. More care must, however, be taken to protect them from light than would be necessary for ordinary P.O.P. or albumen paper.