The Optical Magic Lantern Journal (June 1891)

If Tho Optical Magic Lantern Journal and Photographic Enlargor. 12! The Optics of the Lantern.* By J. TRAILL TAYLOR. I SHALL assume the existence of the light which, for the sake of simplicity, may be that emitted from incandescent lime. Now, as its rays radiate in every direction, the first consideration is the collecting as many of these as possible, and causing them to travel in just the direction we desire. If onlya few of them, ian other words, a narrow angle of illumination, be our requirement, the problem of their forward transmission is one of extreme simplicity; but if, on the other hand, the greatest possible angle of illumination be the desideratum (and of course this is the case), then does the problem become slightly more involved. Wecannot here institute a comparison between the methods adoptei by lighthouse engineers in utilising all their light and those dem inded by the lantern, for in the latter case it is to be employed in the formation of an optical imaze, whereas in the former it is merely projected intospace, and both direct light from a radiant, together with that reflected from either a holophotal ora parabolic reflector, are available. But while we cannot utilise so much of the light as in the lighthouse, we can use it to much greater advantage for our spzcial purpose. We now inquire the greatest angle of light possible to be got advantageously through a condensing system, for this lies at the root of the whole matter, and in doing so have to ascertain how near can the light be approsimated to the first surface with safety. From innumerable trials with lensea of a thickness not too great and set with such a degree of loosenesi in the bras3 work as not to be cell-bound, I find that two inches may be considered as quite safe. When condensers crack it is usually the result of ‘their being too tightly barnished in their cells, brought too suddenly under the influence of the heat of the radiant, or being subjected to currents of coldair. I assume, of course, the perfest annealing of the glass of which they are formed. When this last point is dubious, it may be well to adopt the system employed by the Scotchwomen of a former period, who, when laying in a supply of tumblers intended for whisky toddy, invariably placed them in cold water, which was slowly brought to a boil, and then allowed.to cool slowly, by which treatment : immunity from subsequent breikage was believed to be ensured. At this stage we proczed to analyse the functions of a | lantern condenser—so-called. We find that these are (Ist) the collecting,and (2nd) the condensing of the ! light. Of these the former is much the more important, What we wish done is the collection of so many rays as to form a large angle, and their projection forward in as near ao approach to pirallelism as possible. parallelism cannot pe obtained unless the flame were a point, insteal of being as it is,a disc or patch having sensible dimensions of. say, quarter of an inch upwarda, . Some of the cheap French condensers (of which I would no speak disparagingly, for they render excellent service and are marvels at their price) transmit an angle of light of from 40 deg. to 50 dey., and a higher. class of London-made articles claim, | understand, to embrace 60 deg. than doubles the intensity of the illomination. see in what way this is to be accomplished. : Kepler's law, as you all know, is that the focus of a p!ano-convex lens equals the diameter of the sphere of aonvexity, This is, of course, for parallel rays, and it is *}éx-Com. Camera Club. Absolute . But, by a slightly increased expenditure of optical means, it is possible to increase this. angle to! 5 deg., which, I need scarcely observe, samewhat more ! Le: us | those we are dealing with at present; and we are also | dealing with plano-convex lenses, these being the best : for condensers, subject perhaps, to a slight hollowing of the flat surface, of which I shall soon speak. Well, it is , very evident that if we desire a large angle of light, the j single Kepler won’t do much for us, unless, indeed. it was made enormously thick—even hemispherical—when we would encounter two evils; first, the enormous spherical aberration consequent upon transmi<ting light througha bull’s-eye ; and secondly, the proximity of the said bull's-eye to the radiant, which not only emits light but heat—a heat which would quickly cause our bull's-eye to see stars and stripes. How, then, is it to be accomplished ? By borrowing the ideas of the microscopist. Did any of you ever hear of a microscopic objective of even ths most distant pretensions to wide-angle being composed of one lens? Can you conceiveof such a thing? Well, no more is it possible in our collecting system, which is analogous. We must have, at least, two lenses for our purpose. One of them—that nearest to the light—must be 44in. in diameter in order to catch up the 95 deg. of which I spoke. But this cannot render the rays parallel ; still it transmits them to its colleague under such circumstances that it does so. the two lenses thus doing what no one singly could effect. ; The first lens of the collecting system is comparatively thin, which, apart from any optical advantage, is useful in this respect, that it has to bear the first impact of the heat; and, as you can readily understand, the lessens the liability to fracture. It is only sixteen m.m. (3 inch) thick in the centre, is eight to nine inches focus, and is formed by preference of flint glass. The second element is five inches in diameter, and the radiua of curvature being rather shorter, this combined with its greater diameter, causes it to be proportionally thicker, being twenty-eight m.m. (ly inch) at its centre, and seven inches focus. This lens, too, should be made of colourless glass. ‘I'he loss of light from absorption is but little, and I anticipate an objection that might be suggested as to this from oblique incidence. This is really so little as to be unworthy of notice, but it carries with it its compensation ; for it occurs most at the thinnest portionsof the lens ' where there is the least absorption, and thus aids in ensuring uniformity of illumination throughont the entire beam. But it may be reduced by rendering the first surface concave instead of plane, and retaining the balance of power by grinding the back surface on a tool of shorter radius. I at one time was madly in love with the meniscus form of lens for this purpos2, but incited therethere to by the experieace of Dr. H. Morton, after many trials with lenses both plano and meniscus, and formed of different kinds of glass, from St. Gobain's crown to English flint, I arrived at the conclusion tbat the plane surface answered every purpose in an effective ' manner. If the radiant were infinitesimally small, a parallel beam of a large collected angle could be transmitted with a singular degree of perfection for several yards. Witha : triple collecting system (that worked out by Dr. Charles ; Cresson, in which the first lens is a plano-convex 4iin. radius, the second a meniscus, respectively 30in. and bin.; and the third a crossed lens of S2in. and 8fin. radii) I projected across my bedroom a very tiny gaslight on to the dial of a French clock, which was thus illuminated a whole season. But such extreme nicety ii not required in the practical working of the optical lantern, ae, owing ! to the magnitude of the flame, two elements answer every ! purpose. .The two that I have described shvuuld be | mounted together as closely as possible, fixed permanently in the lantern, and mustalways be used together, and not ssparate, Until a compound collecting sys:em of this nature is tried, one can form no idea of the capabilities of