The sensitizing solution which serves to produce the verifiable image is prepared as follows: Bichromate of ammonia is dissolved in water till the latter is saturated; five grammes of powdered dextrin or glucose are then dissolved in 100 grammes of water; to either of these solutions is added 10 per cent. of the solution of bichromate, and the mixture filtered. The coating of the glass takes place immediately afterward in a dark room; the coated sheets are then subjected to a heat of 50° or 60° C. (120° to 140° Fahr.) in a small hot chamber, where they are laid one after the other on a wire grating situated 35 centimeters above the bottom. Care should be taken not to introduce the glass under treatment into the hot chamber before the required degree of heat has been obtained. A few seconds are sufficient to dry each sheet, and the wire grating should be large enough to allow of the dried glass being laid in rows, on one side where the heat is less intense. For the reproduction of the pictures or images a photographic copying frame of the size of the original is used. A stained glass window being for greater security generally divided into different panels, the size of one panel is seldom more than one square meter. If the picture to be reproduced should be larger in size than any available copying frame, the prepared glass sheets are laid between two large sheets of plate-glass, and part after part is proceeded with, by sliding the original between the two sheets. A photographic copying frame, however, is always preferable, as it presses the glass sheets better against the original. The original drawing is laid fiat on the glass of the frame. The lines where the lead is to connect the respective sheets of glass are marked on the drawing with blue or red pencil. The prepared sheets of glass are then placed one after the other on the original in their respective places, so that the coated side comes in contact with the original. The frame is then closed. It should be borne in mind that the latter operations must be performed in the dark room. The closed frame is now exposed to light. If the operations are performed outdoors, the frame is laid flat, so that the light falls directly on it; if indoors, the frame is placed inclined behind a window, so that it may receive the light in front. The time necessary for exposing the frame depends upon the light and the temperature; for instance, if the weather is fine and cloudless and the temperature from 16° to 18° C. (60° to 64° Fahr.), it will require from 12 to 15 minutes.