This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
A convenient homemade printing device, or distance marker, for printing photographs by artificial light consists of a smooth board on which twelve 1-in. marks are drawn, as shown. A wall-base electric socket is attached on the first line and the others are numbered up to 12. A trial test of a negative marks the distance and time of exposure which should be recorded on the negative. Such a device makes uniform prints possible and provides a means of recording time on negative-storage envelopes. - Contributed by-Harold Davis, Altoona, Pa.
Ill: The Same Distance with the Same Exposure will Always Produce Uniform Prints
Procure a small oval or rectangular frame of a suitable size and use it as a pattern in making a mold. If it is not necessary to select an expensive frame, one that is straight without any floral designs is the best to use. Ordinary molding made into a frame will do as well, or a pattern, whittled out of wood in oval shape, will produce good results.
Make a flask out of any small box, and fill it with clay instead of molding sand. Make an impression of the frame in the clay, and the mold is ready for the plaster.
Procure four 8-oz. bottles, fill them with water, and tint the water in three of them red, green, and blue, with dyes. When purchasing the plaster of paris - 2 lb. will do - also get some brass filings from a machine shop, and mix it with the plaster while in a dry state; then divide the lot into four parts of 1/2 lb. each, or equal parts.
Use the tinted water to mix the plaster and pour it into the mold. This will give the combinations red, green, blue, and white.
Picture frames made in this manner will stand enough polishing to keep the brass filings on the surface bright and shining, which gives a pretty effect. - Contributed by J. B. Murphy, Plainfield, N. J.