In the dusting-on process of photography, a glass plate is coated with a mixture of sugar and dextrine, and sensitised with bichromate of potash, the object being to produce a film that will lose its tackiness or stickiness on exposure to light, the loss being greater in the parts covered by the denser portions of the negative. Thus a positive image can be obtained from a transparent positive only, or a reversed negative from a negative. The process is chiefly used in photo ceramic work, although it provides also a valuable method of introducing fancy backgrounds into portraits, etc. After exposure (ten to thirty minutes in diffused light) some finely divided powder is brushed lightly over the sensitised surface, the powder adhering to the sticky portions. The development can therefore be controlled to almost any extent, and local intensification and reduction can be carried on simultaneously. The process requires some experience in order to secure the best results, and the exposure is very difficult to gauge; an actinometer is used, but atmospheric changes have great influence on the result. Prepare the following.

Grape sugar, 1/2oz.; dextrine, 1/2oz.; bichromate of potash, 1/2 oz.; water, 10 oz.

Whilst this solution is filtering, clean some glass plates, coat them, and dry them slowly over a spirit lamp. Expose as above directed, and allow the plate to stand aside and absorb some moisture from the air. Dust over the powder, and coat with collodion as a protective varnish. A good washing in water serves to remove the bichromate salt.