The Optical Magic Lantern Journal (June 1895)

106 The Optical Magic Lantern Journal and Photographic Enlarger. In theory the one lens forming the half ofa condenser, which is nearer the light, collects the rays from a point, makes them parallel, and transmits them to the second half which condenses them ‘on to the objective, as shown in Fig. I. It will be seen that the nearer the light is to the condenser the more rays will be included; but the light cannot be placed much closer than | three inches on account of the heat, which would crack the glass, also because of the spherical aberration resulting from the thick glass necessary to parallelize the rays with such a short focus. The form (A) does very well when the light is large, as from an oil lamp, and the objective placed comparatively near ; and it is the cheapest effective pattern that is made. (B) is more suitable for objectives of longer focus, the con‘cave form lengthens its focus, gives it greater B 2 Z Z Z Z Z ff depth, and is supposed to collect a little more light than the plano of (a) Fig. II. (c) is a still better form for long focus objectives, its only disadvantage being that its outer curve is liable to get scratched by the slide-carrier or rolling curtain. (bd) nearly resembles it in outward appearance, but the outer curve is flatter which | decreases the spherical aberration, and as made by Dalmeyer this condenser is a much superior instrument to many generally sold, as indeed it sers, and for this reason it enables the first lens to be made much thinner and less liable to break from the heat; the light may therefore be placed as near to itas two inches. The danger of breakage is lessened by the intervention of a piece of glass which in some measure acts as a shield to the leas. A triple condenser certainly transmits more light than a double one, but it is an open question whether the gain in the light is worth the extra expense, especially as they have to be made larger to get the best results. The length of focus required in the condenser is in direct ratio to the focus of the objective, i.¢., the longer the focus of the objective the longer must be that of the condenser. Lanterns are often supplied with objectives of several lengths of focus to enable the operator to show different size pictures without moving the lantern. In theory the condenser should be changed each time, but in practice if the focus of the condenser is suitable for the longest focus ought to be, considering its relative price. Many | the screen. authorities recommend the use of triple conden| objective it will answer fairly well for the shorter ones, but not vice versa. The proper size of the condenser has no relation to its focus, but depends upon the size of the lantern slide. If the condenser is too large much light will be wasted, and if too small the whole of the picture will not be shown. Lanterns are generally fitted with 4 in. condensers, and these would be large enough for the square openings in lantern slides, measuring 32 in. from corner to corner, if they were placed close to the condenser and exactly centered ; but as we shall presently see itis one of the difficult things in lantern manipulation to do this, and, further, there is the thickness of the slide holder, and in the case of double and triple lanterns the rolling curtain ; both of these require a certain amount of space which necessitates the slide being placed nearer the smaller end of the cone of rays, consequently its corners are often cut off. It is therefore advisable in the case of double or triple lanterns to have 44 in. ones, and there is little or no objection to this size in single lanterns, especially as many are now made which permit the slide holder and objective to be moved away from the condenser and so enable the whole of the available light to be utilised. It will be seen from Fig. III that at least 3 of the light collected by the condenser is wasted ; there seems no help for this except to bave circular pictures, but these do not suit all subjects. Tur Ousective. The use of the objectiveis to take up the rays after they have received the image of the p‘cture and bring it to a focus on In the carly days of the lantern this was done, or attempted to be done, with a single lens, and when a short focus was required two or three of these were placed close together with a diaphragm or stop in front (Fig. IV.) There are several objections tothis form, no single lens can be made to project straight lines near the margin, and unless the focus be long the spheri | cal aberration is very great, whilst the stop used : to lessen this cuts off much light. When painted | slides were produced by the artist without the Fig. IV. Fig. III. Fig. V. aid of photography he could compensate for the curvilinear distortion by making the lines of the slide curve in the opposite direction but except with the best class of hand-painted slides sharpness of definition was not a thing to be desired. Now that the majority of slides are produced by photography, and it is necessary that the