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.
Illuminating Objects When Working Against The Light. The manufacturers of engines, locomotives, printing-presses, steam-pumps and other heavy machinery, usually, after preparing all the parts for these machines, assemble them together in the shop. Most work-shops are built with windows all along the sides and with the lathes and other working machinery lined up along the windows, and when machines are assembled they are usually located more in the center of the room. If there is no skylight, the illumination is naturally very poor. The space surrounding the object is generally quite crowded, and there is no opportunity for making the exposure from the window-lighted side; therefore, the shadow side is the only point from which the object can be photographed. To illuminate the object from this side is apparently quite difficult.
Artificial Light. There are two successful ways of illuminating such subjects. The simplest and best way is by means of magnesium-powder alone, a large canvas being placed back of the object and the work entirely done with artificial light. Flashlight is not, as a rule, always successful, as the exposure, being instantaneous, gives bad shadows. Of course, one could use two machines, one on each side of the camera, yet you are apt to meet with false lights; but, by means of the magnesium blow flash-machine, with which you can move the light about from one side to the other, if necessary, you can obtain the same effect of illumination as with daylight, and in much less time. (See illustration of Prosch Magnesium-machine, Page 231.)
Daylight. When one is not provided with artificial illumination and daylight must be employed, it will require some curtaining and reflecting of the light, and this may be accomplished as follows:
222. First, suspend at least two large sheets back of the object, between it and the windows; next, stretch a large sheet in front of the object, facing the light, and on one side out of range of view of the camera. This latter sheet is used as your reflecting screen and should be arranged at a slight angle to the floor so as to reflect the light slightly upward against the object. If arranged perpendicularly the light would very likely be reflected down toward the floor.
Operating The Screens. To operate the screens successfully will require several assistants. While the reflecting curtain remains stationary the others must be adjusted from above; therefore, with one assistant at each end of the curtain, you are ready for the exposure. First, obtain a good focus, using a one-size smaller stop than is necessary for good, clear definition in all parts; then, with the curtains raised just high enough so the light from the windows does not shine into the lens, uncap the lens for making the exposure.
224. At intervals of three to five minutes drop the curtains to within a foot or so of the top of the object being photographed, and then to overcome any halation the curtain should be kept in motion constantly, raising and lowering within the radius of a foot or more above the object. Continue this movement for about one minute at a time, then raise to the normal position for another five minutes. Usually, unless there is some general illumination in the room, such objects will require from 10 to 20 minutes exposure, and they should then be developed by Special Development, according to instructions given in Volume II.