The Optical Magic Lantern Journal (March 1891)

go The Optical Magio Lantern Journal and Photographic Enlarger. ALL applications for space at the Crystal Palace National Photographic Exhibition, in April next, must be sent in by the 23rd inst. The optical lantern entertainments, which have proved so attractive, will be rendered more popular and interesting than hitherto by the addition of | several novelties. J = 2 A CORRESPONDENT takes objection to the paragraph in our last ‘Notes’? in which we made reference to lanterns in churches, by sending us a post-card with the paragraph pasted thereon, and marked ‘ God forbid.” ® * * Tue London and Provincial Photographic Association will give a lantern evening on 3rd inst, at the Champion Hotel, Aldersgate-street. Songs, music, and the lantern, will be included in the programme, which will be under the management of Messrs. Bridge and Freshwater. 10: Triple Lantern Condensers, THe subject of lantern condensers must necessarily prove of the highest interest to lantern exhibitors, because on them depend the intensity and evenness of the illuminated disc. This topic formed ! the themeofa lecture delivered before The Lantern Society by Mr. J. Traill Taylor. He commenced : by analysing the functions of condensers, dividing | them into the two properties of collecting light and condensing it. The collecting element in the condenser he stated to be the most important one; its functions should be to include the greatest nossible angle of light and transmit it to another | ‘ens by which it was condensed to the objective. It is easy enough to have a collecting element “ormed of one lens provided the light was situated at aconsiderable distance from it, but in this case very much of the light would be wasted by not being transmitted to the screen. A safe distance of the radiant had been stated to be from 2#in. to xin. from the first surface, but the lecturer had iad a very powerful limelight burning for nearly two hours at adistance of 2in., which he assumed tc be a perfectly safe one. In order to include a wide angle of illumination, it was necessary that alens 43in. in diameter should be the first element of the collecting system. It was obvious, however, that parallel rays could not be transmitted by a single lens placed at such a distance, for to lo so would entail a thickness of glass so great as to be altogether dangerous, besides which the inargin of such a lens would be unable to transmit the light. To meet this it was necessary that the collecting system should be a binary one at the least. This enables the lens nearest the light to be made correspondingly thin, and also to transmit a very wide angle of light up to its edges. The rays then pass divergingly through it to the second element of the collector at a much smaller angle, and is by it transmitted throughout its whole aperture. The diameter of the second lens should be sin., for if the whole system were made smaller than this it would entail the necessity for the light being brought so near as to endanger the safety of the first lens. The light now passes in a nearly parallel form on to the condenser proper, which is composed of a single lens of any suitable focus of the form best adapted for bringing parallel rays to a point. A condenser having a triple collecting system was exhibited, in which the aberations were so carefully corrected that with a small light, a parallel beam of many feet in length could be transmitted without losing its cylindrical shape, but he thought the triple collector entailed an unnecessary expenditure of optical means to secure a gain that was not commensurate, for the radiant in a lantern was not a mere point, but possessed an area as large as the end of his finger, or, say, a threepenny piece. For this reason he considered that the balance of advantages lay in a double instead of a triple system of collectors. He sketched on the blackboard the two different collecting systems, traced the rays of light through them, and gave the curves requisite for their construction for various kinds of glass. One advantage of this condenser was that, in-addition to transmitting an amount of light very much in excess of that obtained by any of the condensers now procurable in commerce, the light could be employed much more effectively for such scientific purposes as polarizing than by any other means. The angle of polarization was a definite one, and that could not be secured in an effective manner by rays which converged on the bundle of glass plates that served as the polarizer in the lantern. He described the means by which he had effected this by the intervention of a concave lens to render the converging rays parallel. He had brought this before the British Association for the Advancement cf Science over twenty years ago, and it was received by them with much approbation. But the American system of transmitting the rays ina parallel beam from the collecting system of the condenser was in his estimation superior to his own. Having secured the maximum of light, the next question was its condensation through the picture and upon the object glass of the lantern. He recommended that the condensing lens should be easily separable from the collecting system,