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.
Daylight. After a long series of experimenting with various kinds of light we have become satisfied that for the amateur-whose enlargements are only made occasionally-the daylight method is the best to employ. Daylight requires no extra apparatus, as any ordinary view or hand camera is all that is necessary, whereas for artificial light more preparation is required. One of the drawbacks to daylight, however, is that many do not have the time to work during the day and must make their enlargements at night-hence the necessity of using artificial light.
617. Another drawback to daylight is that it varies in intensity, rendering the timing of exposure somewhat difficult. It is, of course, weaker in the early morning than in the middle of the day. Possibly, too, while making the exposure the sun becomes obscured by a cloud. This objection, however, you will readily overcome by always trying a small test piece of paper on each subject before making the full size print.
618. When using daylight, you must provide yourself with a good sized reflector, so that the light will be reflected evenly on the ground-glass. This should be arranged on the outside of the window, so as to throw the light coming from above onto the ground-glass. To ascertain whether this is in the proper place, take out the lens and examine the ground-glass from inside the dark-room. If the ground-glass appears evenly illuminated your reflector is properly adjusted; if the sun shines on ground-glass and reflector, providing it strikes evenly, it will do no harm. If the light is too strong the lens can be stopped down. You will find that a mirror will produce strong reflection, making the light much more powerful; and in case of using a lens of small aperture, this would be desirable, as the mirror will reflect the rays of light from the sky and give a more even, uniform, and stronger light than any other reflector. If the sun strikes the mirror it should be covered with white paper.
Electric Light. Where condensers are used electric light will give a stronger light than is actually required. You may diffuse this light by placing a piece of ground-glass between the condensers. The slight loss of light caused by the ground-glass is no detriment. When the Folmer and Schwing, or the home-made outfit, with the cone and ground-glass (described later) is employed, you will produce softer results and you can diffuse as much as you like with additional ground-glasses. All that is required when you want more contrast is to use a less number of ground-glasses.
Gas And Kerosene. While gas and kerosene are hardly powerful enough for enlarging, except on a small scale, very nice results can be produced, but longer exposures will be required of course.