The Optical Magic Lantern Journal (February 1894)

The Optical Magic Lantern Journal and Photographic Enlarger. 31 lavished upon its construction and fitting ; but. in later and more modern patterns much depends upon the personal skill of the operator. Many of the recent improvements, although very valuable in themselves, are possessed of this drawback :—they have made the lantern more complicated, and, while giving an increased command over the apparatus, have by no means simplified the working of it. Under these circumstances some acquaintance with the optical system of the lantern is not only desirable, but is absolutely necessary for those who are anxious to do justice to such an intricate and delicate instrument as a high-class lantern has become. Anyone who is really master of his apparatus who not only can produce from it the very utmost it is capable of, but who also has, so to speak, a reserve fund of knowledge wherewith to meet and combat possible breakdowns on the spot, will take more pleasure in its use, and derive more satisfaction from the display of its qualities. In the following article an attempt will be made to explain, for the benefit of the inexperienced, in a simple, and so far as the subject will allow, in a non-technical fashion, the work performed by the various lenses which form the essential parts of a lantern. The subject is not, perhaps, a very interesting one, but the actual benefit to be derived even from a rudimentary knowledge of the properties and capabilities of the different optical parts amply repay one for the outlay of time expended on it. ‘he result which an ordinary convex lens, that is to say, a lens which is thickest in the centre, has upon rays of light is, as is well known, to converge them to a point; while, on the other hand with a lens with concave surfaces, the light is diverged and spread. Why this should happen can be popularly explained by means of the followng particularly unscientific illustration. Imagine a number of soldiers, linked arm in atm, advancing forward in a line at, say, four miles an hour, The men shall represent a wave of light emitted from a luminant such as the lime light. Now, if an oval-shaped pond of shallow water be interposed in the line of march, let us consider what will happen. As the line of men enter the water, owing to the increased density of the medium through which they are marching, their pace will be checked to, perhaps, one mile an hour. The pond being somewhat oval in shape (or as we should, say in speaking of the section of a lens — by-convex) it will result that the centre portion of the line will have a greater distance to travel through this denser medium, and that consequently this centre portion will be most retarded, and will be still wading in water at one mile an hour when the two ends of the line have already passed through the narrower extremities of the pond. Remembering that the men are supposed to be linked arm. in-arm, it will be seen that when the whole line has issued from the water, it will be more or less semicircular according to the width of the pond. But what will happen if the march be continued? As each man will walk straight to his front, that is to say, in the direction in which he is now facing, it is evident that the two ends of the line will converge together and eventually will meet at a point ahead of and midway between them ; the distance of this point depending on the amount of curvature of the line on leaving the water, or, what comes to the same thing, the curvature of the pond. If the pond, instead of being bi-convex in shape were planoconvex, that is to say, one bank straight and the other curved, the line of men on issuing would be less curved and more nearly approaching a straight line, and consequently the point at which the men would meet would be further off. This illustration of a lens’ action is, perhaps, rather childish, and, of course, inaccurate, but by looking at a lens in this light, and by regarding the glass of which it is formed, merely as a denser medium which retards to a greater or lesser degree according to its thickness, the ray of light which passes through it, we are able to roughly guess, off-hand, what the action of any given Jens, or combination of lenses, will be. We shall now be in a position to understand something of the principle on which a condenser works. As its name suggests, its action is to condense the rays it receives from the luminant ; but equally it is a collector, gathering up rays from the light which otherwise would be lost, or, at all events, uselessly dispersed. Except in toy lanterns the condensing system is never single but is always compound. Not only would the condenser, formed of a single lens, be too thick to stand the heat, but also its focus would not be of convenient length. By dividing the condenser into two parts not only will each lens be thinner and less likely to crack when greatly heated, but also the combined focus will be shorter, and by thus pemitting the lime to be brought nearer, a greater angle of light will be included. This matter ,will be referred to a little later on. We will select for consideration the most ordinary form of condenser, known as the double plano-conyex, and which may be roughly described as consisting of the two halves of a