LUMINOUS PHOTOGRAPHS Photographs which appear luminous or phos phorescent at night. They may be prepared in several ways. The simplest is that of making a positive transparency—as, for example, a lantern slide—varnishing the film side, and coating with luminous (Balmain's) paint. When dry, the transparency is backed with thin wood or cardboard, with a ring for hanging, and the whole bound together with strips of gummed paper or cloth. If exposed to light during the day the picture will appear luminous at night. Another plan is to spread a thin coating of glue upon cardboard and sprinkle with powdered barium or calcium sulphide, or to coat with Balmain's paint. A print is then made upon thin sensitive paper which, after finishing in the usual way, is made transparent with castor oil, the excess blotted off, and the print attached to the treated cardboard with thin glue or strong paste, and dried by heat.
A method popular in Germany is to coat a piece of thin transparent celluloid with the following : Gelatine . . . 436 grs. ioo g.
Potas. bichromate . 48 ,, I I „ Calcium sulphide . oz. 55 ,, Water . . . io „ i,000 ccs.
The gelatine is soaked in the water, melted by heat, the other ingredients added and dis solved, and the mixture filtered through cotton wool. When dry, the coated celluloid may be printed upon from a positive through the celluloid film, and the image developed in warm water in the same way as in the carbon process, a print being obtained which shows as a negative by transmitted light. This needs to be backed with black velvet or black paper, when it will appear as an ordinary black-and white positive by daylight, and as a luminous or phosphorescent picture by night.