The Optical Magic Lantern Journal (November 1890)

so condensers, Mr. J. Traill Taylor showed by diagrams that in the case of a perfect optical system there was no advantage in a large objective, provided that the object in the slide was clear black and white. With photographic slides, however, onaccount of their semitranslucency, there was a good deal of light passing that was dispersed so far as to be out of the area included by a small objective; whilst with a large obiective this light would be received and utilised in the formatio 1 of the large image. A form of objective which he had constructed, and which worked very successfully, was a slight modification of the ordinary Petzval portrait lens. In order to obtain greater flatness of field he had used a double convex of about equal radii for the positive lens of the back combination. This, with a suitable concave lens and a small front lens, gave very satisfactory results. With a_ paraffin illuminant there was an advantage ina large objective, as well as to catch the light spread about by imperfection in the condensing system. As an example of what could be done, the late Mr. Thomas Grubb had constructed an achromatic condenser of 6in, diameter, capable of passing the whole of the light through a gin. microscopic objective. It was impossible to make a perfect condenser with less than three glasses. There were many inch condensers that would not illuminate a 3in. slide, especially if of cushion shape. He had overcome the difficulty on one occasion by unscrewing the condenser till it only just hung in its place, and so brought it close enough to the slide to illuminate it all over. Following up the question, Mr. G. R. Baker, in an article in the Brrtish Journal of Photography, asks : “Shall condensers be of various foci for different objects, or will one do for all purposes ? Theoretically, each objective used should have a condenser to suit its particular focus; but, practically, it will be found when using the limelight that a good compound condenser of about 33in. combined focus will answer for all objectives from 6in. to 12in. focus. The form: of lenses is not of so much moment as their correct working and true centring, but I prefer a meniscus back and double convex front for limelight work, and stze, qtin. diameter. The back lens may be a din. plano if preferred, and the front a 4}in. double convex. For the oil light, where the flame is large, and a qin. condenser employed, the two plano-convex lenses of 6in. focus, each mounted with their convex surfaces nearly touching, and giving a combined focus of 3in., are as good as can be desired, and the flat surfaces being outwards permit of the lens coming close up to the slide—a very important point, for it is only by this means a cushion-shaped slide can be well covered and projected by a shortfocus objective. “ The Condenser to move to and from the Slide.— | This, in my judgment, is an adjustment that should be provided in the better class of lanterns, for the cone of light can be made by this means to just cover the photograph, and so concentrate all the rays ; whereas, if stationary, a considerable number would be wasted when showing circular pictures if the distances were arranged for square or cushioned matted ones. The slide can be moved forward by an adjustment of the stage, or a double stage can be arranged, the same as was done in a ‘ Bridgman’ triple lantern made for the Rev. C. H. Turner some years since ; or, what perhaps is a better plan for solid-fronted lanterns, the condensers can be moved in their tubes slightly by means of The Optical Mazic Lantern J ournal and Photo graphic Ealarger. bayonet catches or a spiral adjustment. This latter method has been very well developed in a triple lantern recently made for Sir David Salomons, for a ready and very convenient adjustment is obtained by simply touching some non-conducting heat knobs attached to the cell of condenser, the spiral allowing of the most delicate movement. It will be readily conceded that an adjustment of this kind could not be made continually during an evening's entertainment, for it would involve manipulating the light in the latter case. or refocussing in the case of the double stage; but if all square or all round pictures were to be shown, the adjustment could be made to suit either at the commencement, and so ensure the best light possible. “ Objectives .—Theoretically, if one is using a point of light, or a perfect light, asit has been termed, small lenses for the objectives answer, providing the condenser is of a focus to converge the rays so as to fully illuminate (after passing through the photograph or picture) the lens when at its proper focus. In practice one does not get a point of light, for with the limelight the incandescent area is comparatively large, and with the electric arc light there is a certainamount of fame caused by the combustion of the carbon. Hence the rays coming from the luminant and passing through the condenser are not all focussed together, and consequently there is a secondary area of illumination which, in the case of long-focus objectives, is of considerable moment. It is well-known the lime or luminant has to be nearer the condenser when the long-focus lenses are used, and the rays are then approaching towards the parallel ; consequently those straying from the true path, after converging through the condenser, will be lost, unless the back lenses are large. “It is easily proved how much stray light is coming from a large luminant by using a pair of 2in. planoconvex achromatic lenses as the objective, say, of 6in. equivalent focus, with a three or four-wick lantern, and unless a small top is inserted in front of them, it will be found the projected picture on the screen is all fuzzy ; while with the identical lenses in the same position when using the limelight, it is quite clear and sharp. Now if the rays from the oil-lamp and condenser are received by a correcting lens of 2in. diameter, which is nearer the slide, and then passed on to the achromatic front lens of the combination—on the same plan as the portrait lens—the resultant picture on the screen will be satisfactory. The limelight works, when set for long-focus objectives, somewhat in the same way, although ina lesser degree, and unless a large-diameter lens—say, 3in. for 12in. or longerfocus objectives—is used for the correcting lens, all the rays are not gathered up and passed on to the achromatic lens. For objectives of 7in., 8in., and Join. focus the size recommended is 23in.diameter.and for the 6in. focus, 2in. diameter. Iflarge-diameter single lenses —say, 2} or 24—are used of the plano-convex form of short focus, such as I have seen sometimes in apparatus, the result is generally bad definition, no matter what size stop (within reasonable limits) is used. This applies to those of 7in. and shorter focus; therefore, nothing larger with this form than 2in. should be used; and then better results are obtained by combining two longer-focus plano-convex lenses to give the same focus—say, two I4in. to give 7in., or Ioin. to give 6in. focus. The very short-focus ones require special correcting.”