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.
The ordinary hand camera of the focusing type can be used to enlarge pictures from negatives of its own make. The requirement is a device to hold the negative rigid in a position in front of the camera lens, and at such a distance that the rays of light passing through the negative and lens will enter a box of sufficient size for the desired enlargement and focus plainly on a sheet of sensitive paper attached to the end of the box.
The first thing to do is to find the distance that is required from the camera lens to the paper enlargement to make the proper size, and the distance from the lens to the negative. A correspondent o f Camera Craft gives the following rule for finding these dimensions: To find the distance between the lens and paper enlargement, add
1 to the number of times the picture is to be enlarged and multiply the result by the focus of the lens in inches. The example given is for a 6-in. focus lens. An example: A 4 by 5-in. negative enlarged to 8 by 10 in. is a two-time enlargement (four times in area) ;
2 + 1=3, and 3X6=18, the distance in inches of the lens from the sensitive paper. To find the distance of the lens to the negative, divide the above result, 18 in., by the number of times desired to enlarge, 18/2=9, the distance in inches from the lens to the negative.
With these figures as a working basis, the box can be made in any size to use any focusing camera. The dimensions given in the drawing are for a 4 by 5-in. camera having a 6-in. focus lens, and to enlarge the pictures from a 4 by 5-in. negative to 8 by 10 in. In the first place make a box 8 1/2 in. wide, 10 1/2 in. deep and 14 in. long, inside measurement, using 3/4-in. material, as shown in the sectional drawing A. One end is left open and in the center of the other a hole is cut 5 in. square. The back end of the camera is placed over this hole as shown at B and 1/4in. strips nailed to the box end around the camera back to exclude all light. The camera must be centrally located. The next to be made is the end board or easel, consisting of two pieces of 3/4-in. material, one 8 1/2 by 10 1/2 in., which should fit easily into the end of the box, and a larger one, 10 by 12 in., the outside dimensions of the box, as shown at C. Nail the smaller piece to the center of the large one, crossing the grain of wood in so doing. The end board is the easel upon which the sensitive paper is fastened with push pins, and should be covered with a sheet of white paper, pasting it on the 8 1/2 by 10 1/2-in. board with a thin coat of glue. The slide D is a piece of wood 3/4in. thick, 3 1/2 in. wide and 26 in. long. This is fastened to the under side of the box with four screws, placing it exactly in the center and parallel with the sides of the box. Be careful to have the slide parallel or the holder will not freely slide upon it.
Ill: Details of Construction and Camera Complete
The negative holder E is made of a piece of 3/4-in. board, 8 in. wide and 10 in. long. A hole 5 1/2 by 7 1/2 in. is cut in its center, leaving a margin of 1 1/4 in. on all sides. This holder is set in a groove cut in a block of wood having a mortise cut 3/4 by 3 1/2 in. to fit on the slide easily. A thumb screw is fitted in the center of the bottom of the block of wood. This is used for fastening the negative holder rigidly to the slide when the focus is secured.
A 1-in. hole is bored in the upper corner of the box end, as shown, to serve as a peephole for seeing the image on the end board or easel. This is covered before putting the sensitive paper in the box. The end board is held in position with two flat brass hooks. The camera is held in place with two buttons placed on blocks of wood the height of the camera back, as shown at F. Two pieces of clear glass, 6 by 8 in. in size, are held in place in the negative holder by means of buttons, the film negative being placed between them. All the joints in the box must be carefully puttied and the inside of the box blackened, which is done with a mixture of lampblack and alcohol, to which is added a small quantity of shellac to give it body.
A darkroom is not essential, a bathroom with the window covered over with orange paper will do, or even a large room with the shades drawn and pinned close to the window casing. It is best to leave a space in one of the windows to be covered with orange paper, doing the developing about 10 ft. from the source of light.
To operate the camera place it on the enlarging box, hook the easel in place, put a negative in the holder with the film side toward the lens. Take the outfit to a shady place outdoors, point the holder end at an unobstructed portion of the sky and look through the peephole. Rack the lens in and out to focus the picture. The easel should have heavy black lines drawn upon it inclosing parallelograms from 5 by 7 in. to 8 by 10 in., so that one can readily see the size of the enlargement to be made. When the focus is obtained take the outfit into the darkroom, remove the easel and fasten the sensitive paper with push pins. Replace the easel and take the outfit outdoors again, point it toward the clear sky and make the exposure, which should be at least 5 seconds with a 16 stop. It is best to make a trial exposure on a small strip of paper to find the proper time. Directions for the use of bromide papers will be found in each package.