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 most subjects the latitude of a plate will be about two, that is, if one second were correct exposure, two seconds would not be too much for safety. For beginners an open, sunlit landscape is a good subject for the first experience. Try three exposures 1-100, 1-50 and 1-25 of a second. In some cases all three might be good and in others none, but there will be evidence enough to give a line on correct exposure. If the shutter has but one speed, or is not reliable, the same experiment may be tried by starting at a full open lens and reducing the stop one size each time.
734. The light varies in intensity from hour to hour during the day and from month to month during the year. In winter exposures at noon should be from two to four times longer than at noon in June.
736. On very dull days when the sun casts no perceptible shadows expose four times as long as for bright days. Even at best the picture will be flat, but if under-exposed it will also be weak and thin. If a landscape has dense foliage in the foreground double the exposure.
Developing Light. Ruby glass is the best medium to use in the construction of a light. Some ruby glasses pass light which will fog a plate in short order, but a large percentage of them are safe for all practical purposes. The principal object is to see what is doing. There is least strain to the eyes when the light which illuminates the work is moderately strong but not harsh. A little diffusion through several sheets of yellow tissue paper will secure this quality. It will be found that no light in which the shape of the illum-inant (gas flame, or candle, or incandescent filament) can be distinguished, is a safe or pleasant one to use.
739. There is hardly any light which is safe and at the same time strong enough to be useful. The proper way is to get a diffused red light which is comfortable and then make a practical test to see how soon a plate exposed to it fogs.
740. Put a plate in the plate holder in perfect darkness; then place the holder where you generally develop, draw the slide half across the plate and expose to the developing light as long as it generally takes to develop a negative. Then develop the plate in perfect darkness the usual time, wash and fix. If any difference is then found between the exposed and unexposed parts of the plate, it is proof that the light is not safe for very sensitive plates.
741. It should be pointed out that Orthochromatic plates are very much more sensitive to the developing light than regular plates and therefore require some special care in handling, though they do not necessarily require a special light for their development. The ordinary light will be quite satisfactory unless the development of the fastest ordinary plates by it is risky. After the developer has been poured on, Ortho-chromatic plates are not a great deal more sensitive to red light than ordinary plates. The greatest care should be in handling before development. Keep as far away from the light as possible until the plate is covered with developer.