The Optical Magic Lantern Journal (February 1894)

32 The Optical Magic Lantern Journal and Photographic Enlarger. bi-convex, mounted together with the flat sides turned outwards. In the accompanying figure L is the luminant which we will suppose to be a point of light only, and not, as is the case with all kinds of limelight, an incandescent surface of considerable size. Ais a wave of light from this luminant, advancing forward in a curve similar to a ripple formed by disturbing the surface of calm water. The centre arrow of the three having the greatest distance to travel through the denser medium of the glass is retarded most; the upper and lower arrows least ; and, consequently, the rays, which are curved on entering the collecting element of the condenser, emerge in the straight line B, and are transmitted parallel on tothe condenser proper. Here again the middle arrow denotes the part of the ray which will be most retarded, while the upper and lower ones have but a slight thickness to pass through, and go being out first, the ray assumes the curve marked ©. Advancing in the direction indicated by the arrows, all parts of the ray meet simultaneously, at the point F, the posterior focus. This diagram, of course, neglects sundry important factors, but it will serve roughly to explain what takes place in a condenser. If the front tube, or bellows, of the lantern be removed, experiment will show that the nearer [_ is moved towards the condenser the greater distance will F recede; a slight advance of the former producing a considerable lengthening of the latter. For every position of | there is a definite position of F, and pairs of points thus related to one another are known as conjugate foci. Since the objective, or front lens, is placed at F, it follows that the greater the distance between the condenser and the objective, the nearer the light must be placed to the former. All lanternists are well aware that the same lenses which, with an oil lamp as luminant, only give poor definition, will, when lime-light is used, yield a much better result in this particular respect. If the luminant is a large one, such as an ordinary 3-wick lamp, we may consider it as consisting of a great number of luminous points (instead of, as with an ideal lime-light, a single one), each of which emits its own set of rays to the condenser, and which are duly condensed in cones to points on the other side of the condenser. These cones, being emitted by points which are separated from each other by sensible distances apart, cannot possibly coincide; the images formed will overlap each other, and consequently bad definition will result. To put the same matter another way—the light waves will not be truly spherical but will leave the condensing element in more or less irregular curves, incapable of being all transmitted to one single point. Fig. II., in which the angles, &c., are exaggerated for the sake of making the diagram clearer, will explain still further the action of a condenser and the relation which the position of luminant has to the length of the front cone of light. If the lime be placed at A, the angle of light included by an ordinary plano-convex condenser at C will be about 50 degrees; and the point at which the convergent rays will cross be at A/. If the light be moved to the point B a trifle nearer the condenser, we shall find that the front cone is greatly extended and, as indicated by the dotted lines, will reach B/ before the rays cross. From the diagram it will also be seen that, as before mentioned, a greater angle of light is included when the lime is at B than when at A; and so there is a distinct gain in light by bringing the luminant nearer to the condenser. On account of the great heat evolved there is a limit to the nearness with which a lime can be brought to the collecting lens. Allowing the condenser to warm up gradually, and by shielding it from sudden draughts and paying due attention to the lime, 2 inches may be considered quite safe with properly constructed lenses. By experiment with a small disc on a screen § feet or 9 feet from the lantern, the writer has found that with a 4 inch diameter condenser, consisting of a miniscus lens mounted close to a bi-convex, the actual measurement of these distances are:—With 4 inch back focus objective, lime to condenser, 3 inches; condenser to back lens of objective, 64 inches; with an 8 inch focus objective, the distances are 22 inches and 11 inches respectively. For use with lime light the miniscus condenser mentioned above is preferable to the plano-convex, shown in Fig. II., but with an oil lamp it does not give the best results, and the ordinary plano-convex is more suitable. A triple condenser is formed by inserting a third lens between the double condenser and the