S represents our glass slide of 3 inches clear diameter, R the radiant or illuminant, and L our objective, shown here for the sake of simplicity as a single lens.
The slide is well illuminated by the light emanating from
R, but it is obvious that the bulk of this light will never pass through the lens, and, in fact, only the very centre of the slide will under these circumstances appear upon the screen at all.
What is evidently wanted is to converge these outer rays, or in other words to bend them in so that they also pass through the objective, and this is the function of the condenser as illustrated in Fig. 35. The condenser is here represented also by a single lens, but in practice it also is invariably constructed of two or even three lenses, for both optical and mechanical reasons. It is evident from the above diagrams that the condenser must be somewhat larger in diameter than the slide itself, and condensers for ordinary lantern work are usually 4 inches to 4½ inches diameter. The former size
will suffice if the condenser is placed very close to the slide, but it is often advisable to leave a little intervening space, especially if the illuminant is a powerful one, in order to allow any condensation of moisture readily to evaporate and escape. Hence lanterns for long range work (which involve, of course, good illumination) are usually made with condensers of 4½ inches diameter. Lantern condensers of to-day usually take one of the two forms shown in Fig. 36, but the exact curve must be left to the manufacturer, as the focus of the condenser must have a definite relation to that of the objective. Taking, however, the design of E, the most common of all, the two lenses should not be exactly similar unless the objective is pretty short in focus, or, in other words, unless the distance of the illuminant on the one hand and that of the objective on the other are approximately equal. If the lantern is intended for long range work, that is equipped with a long focus objective, the front component of the condenser should also be constructed longer in focus (that is to say, with a shallower curve) than the rear one, and it is amazing how careless manufacturers are in this respect. If, as is often the case, the lantern is fitted with several objectives of different foci, it is usually necessary to supply alternative condensers also, or at least to supply an interchangeable front component.
If the entire condenser is too long in focus, light is lost; if too short, it is impossible to obtain an even disc, as there is invariably a dark patch either in the centre or round the edges.
The mounting of the condenser also varies with different makers; but it must be remembered in any case that it gets