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.
By W. L. Patterson
Of the Bausch & Lomb Optical Co., Rochester, N. Y.
937. The optical principles of Projecting and Enlarging are practically the same.
Illuminant. In the matter of illuminants for projection we might put aside all lights excepting the electric arc and possibly the lime light. We use an electric arc with carbons at 90 degrees; for alternating current an arrangement with less angle would be better. Fair work in enlarging can be done at short distances with the Welsbach gas lamp, because the sensitive paper is capable of taking up an image which would not be satisfactory for projection purposes.
Adjustment Of Arc. The electric lamp must always be adjusted with the arc central to the axis of the optical parts, both in projecting and enlarging. If the arc is not central with the condensers there is unevenness in the illumination of the field. A great deal depends on the distance between the rear condenser and the arc. If we bring the are close to the condenser with the lens now in the lantern before us, we can quickly see that a great amount of red color is introduced into the margin of the field. If we put the arc too far away from the condensers we get a blue margin in the field, therefore, we have one correct point where the field is fairly well illuminated, and that has been found to be, from general experience, the point where the converging rays from the condenser cross in the projection lens, i. e. the light has to be so placed from the condenser as to bring the focus of light or crossing of rays to where the axis of the projection or enlarging lens is situated. (Mr. Pancoast suggested that smoke from a cigar or other source would render the rays visible.) This brings us to a ques-
Illustration No. 98 tion which we are often asked, that is, what should be the relative position of lamp to lens? This is not constant for all lenses; it will vary with each and every lens and it will vary with the same lens, because the position of the lens with relation to the image changes as the object changes with relation to the lens, and for that reason you should adjust the arc for various distances and enlargements, and also for various lenses. Care should also be taken to keep the lamp in proper adjustment, if a hand feed form, for if the arc flames, it is too long, i. e. - carbons too far apart. If the arc hisses with direct current, the carbons are too close. Alternating current invariably produces a buzzing noise.
Illustration No. 99
Triple System Condenser. In our projection apparatus we have introduced a triple condenser system, Illustration No. 98, consisting of one mensicus and two planoconvex lenses. This system has many advantages over the double condenser system, Illustration No. 99. First, the angle is greater because the arc is nearer the lens and there is thus collected from it a larger cone of rays. The beams are rendered parallel by the two rear lenses, in the double condenser system by the one rear lens only. Second, the meniscus being the rear lens, the arc is at practically a uniform distance from all parts of the rear surface of this lens and there is much less loss of light by reflection. Third, the lenses are thinner and will expand and contract more readily with the heat and cold than the thicker lenses and are therefore less liable to crack. Fourth, we can without disadvantage place a water tank between the lenses of the condenser system to cut off the heat rays, not only for lantern slide projection, but also and principally for microscopical projection. This is also valuable in bromide work because the heat from an arc light is often intense, and when a negative is valuable it is well to use some precaution to prevent injury, and also to protect valuable photographic lenses. In the double condenser system the tank as ordinarily applied in microscopical projection is impractical for use in the projection of lantern slides because, as can be seen from Illustration No. 99, so large a part of the field is cut off.
941. The foci of single condensers are usually as follows:
5 1/2 or 6 1/2
5 1/2 or 6 1/2
5 1/2 or 6 1/2
5 1/2 or 6 1/2