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.
As the negatives must be sized, it is necessary to use films. A camera using films 31/2 by 31/2 in. will make negatives that can be trimmed 1/4 in. on each side to make prints 3 by 3 1/4 in. Each two negatives making a pair are fastened together, properly transposed, by folding a narrow strip of black paper like a long, V-shaped trough, pasting it, and putting one on the bottom of the two negatives, as they lie side by side, and one at the top, saddle fashion. This can be done still easier by using strips of passe-partout binding, or strips used for binding lantern slides. If so desired, the use of black paper can be carried farther by cutting the top strip of binding paper in such a way that it gives the round corners to the top of the prints. A narrow strip through the center and a binding of black paper along the two end edges make a mask unnecessary in printing.