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.
Exposure For Contact Printing. When making a lantern-slide by contact, the exposure should be very much less than when photographing a negative. Proceed to the dark-room, and by the light of the ruby-lamp only, place the negative from which the lantern-slide is to be made into an ordinary printing-frame; next place the lantern-slide plate (or ordinary plate if the lantern-slide plate is not being used), film to film, in the frame, being careful to center the unexposed plate on the negative. Lay a pad or piece of black paper over the back of the unexposed plate; place the back of the printing-frame in position, and fasten down the springs. Then, by the light of a match or any artificial light, expose the plate at a distance of from two to three feet, or at arm's length, from the light.
885. If a 16-candle power electric light is being used in the dark-room, hold the frame about three feet from this light, and expose from one to two seconds, the length of exposure depending entirely upon the density of the negative and the speed of the plate used. If the negative is a slow printing one, it may require a longer exposure; if thin or quick printing, it will require considerable less. If the light of a match is used, it will take from two to three seconds at a distance of one foot from the negative. If the regular fast plates are used, one second exposure is sufficient.
886. This key to the length of exposure applies where ordinary plates are being used. If the regular lantern-slide plates are used, it will require at least double this, as they are much slower.
887. As before stated, an ordinary 4 x 5 slow plate can be used for making the negative, and a regular lantern-slide 3 1/4 x 4 for the slide. After a few experiments, and keeping record of the results, it will be easy to judge the correct length of exposure.
888. Remember that both the negative and the lantern-slide plate must be carefully dusted when placing in the printing-frame. The slide plate should be laid gently on top of the negative, so that the plate will cover that part of the picture in the negative which is to appear on the slide. If the negative is larger than the lantern-slide plate, be careful not to slide the latter about on the negative because if there should happen to be the slightest particle of grit or dirt between the two surfaces, one or both of the films would be injured.
889. Always be careful to have the distance the same from the light, as a little change in the distance of the printing-frame from the light will make a great difference in the length of exposure that the plate receives. Alter the time only to suit the different negatives.
890. A good plan is to place a box or other support a certain distance from the light; drive a nail a trifle above the center, and put a picture hook or screw-eye into one end of the printing-frame, by which it can be hung up on the nail in the box at exact distance from the light.