This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Focusing. Before focusing, see that your camera and negative are in position, and the room in total darkness, and that the only light coming into the room is passing through the negative and then through the lens. By pushing your easel or stand-which is covered with white cloth or cardboard-forward or backward, you can obtain any size enlargement you want. When you have determined this, focus in the usual manner, by either racking your lens forward or backward. If the lens cuts the image perfectly sharp, and a rectilinear lens will do this, as a rule, you are then ready to place your sensitive Bromide paper in position. If you find, however, that the lens does not cut the image sharply, it will be necessary to use a stop. Never use a stop or diaphragm smaller than is absolutely necessary. Next place the cap on the lens. This cap should be fitted in the following manner:
651. Cut out the front of the cap, leaving about 1/4 inch margin around the entire edge. Into this fit a piece of yellow or bright ruby glass. You will find that the image will show on the screen colored yellow or ruby according to the kind of glass you have in the cap. You will also find that the light coming through the colored glass will not affect the paper. After capping your lens attach the paper in position, with thumb tacks, being guided by the colored image on the screen. We would advise the use of Kodak push-pins, as they are made of glass and therefore would not leave so large a white spot on the print. With the paper attached to the board you are ready to make the exposure.
Making The Exposure. Before making an exposure on a full size sheet of paper we would advise using a test strip. Take a sheet of Bromide paper and cut it into strips about three inches wide and experiment with one or two of these to obtain the proper length of exposure. In attaching the test strip to the board place it in a position so as to cover portions of the negative containing highlights, half-tones and shadows. This will give you a fair test on all portions of the negative. In this way you will avoid waste. After you have obtained the proper exposure, make a record of the number of seconds on the margin of the negative, which record will be your guide on all future exposures from this negative. The exposure varies with the density of the negative, as well as with the quality and strength of light. A thin negative with sufficient strength to make a good print generally will receive sufficient exposure in one second in diffused daylight, or twenty seconds at a distance of 12 inches from a No. 2 kerosene burner. Very thin negatives should be printed by weak yellow light; in fact, it is better to print weak negatives by artificial light.
as the artificial light always gives more contrast. In this way a fairly strong, vigorous print may be obtained from a negative that would otherwise be too thin and flat. Thin, weak negatives should also be printed on "hard" paper; strong, intense negatives should be printed by daylight, or, at least, by strong artificial light, and on a "soft" grade of paper.
653. The correct time of exposure depends on the negative, the time and brightness of the day, or, if made by artificial light, the strength and color quality of the light. A good way to determine this is to take one of your test strips, tack it on a screen in the manner stated above, then with an ordinary card, cover all but 1/3 of the strip; take off cap and expose 5 seconds; move the card 1/3 more and give this 5 seconds; then give the whole strip 5 seconds, thus exposing the first part 15 seconds, the second 10 seconds and the third 5 seconds. Develop the strip and one of these exposures will be a good guide to the correct time. Bear in mind that daylight is about twenty times faster than lamp light.
654. The image must be made to register properly on your box or enlarging easel, by sliding the negative in the holder and by raising or lowering the front of the camera carrying the lens, providing your camera is supplied with a rising and lowering front.