The Optical Magic Lantern Journal (June 1891)

122 Tho Optical Magic Lantorn Journal and Photographic Enlarger. the lantern for certain scientific purposes, such as polarising. Let us now direct our attention to the condensing element of this optical system. We have secn that the tivo elements of the collecting portion must be fixed and inseparable. This, on the contrary, should be variable, and selected to suit the special end in view. Its form may be plano-conves, more especially if for nse with longfocus objectives ; but if the latter is to be short-focus, and the condenser of crown glass, then is tbe crossed form, in which the curves are as one to six, or two to thirteen, open to be preferred. But, dealing as we now are with immergent parallel rays, it were folly to imagine that a condenser properly adapted for an objective of }2in. focus will answer equally well for one of Gin. Bearing in mind Kepler's law, which, however, upplies only to one kind of ¢lass, and must not be held as applicable equally to the flint glasses, especially those of the denser sort procurable at the present day, I would say that for long projection lenses of 12in. to 15in. focus a plano-convex having a radius of curvature of 7in. willserve every purpose; for an objective of Sin. | to 10in. the radius may be 4+4in., while for one of Gin. to Sin., din. will suffice. But, as I have said, this latter may with advantage be a crossed lens, in which case the radius ! of the more convex side will be longer. One word more before dismissing the condensers. Treat them with the most scrupulous care, both in heating and cooling, and avoid allowing a current of cold air to play upon them during the cooling. With this precaution, superadded to having them set loosely in their celle, a fracture will rarely, if ever, occur. Turn we now to the objective—the image former. First its diameter, especially that of its posterior combination, must be snfliciently large to take in not merely the whole of the cone of rays emerging from the condenser, but by preference a little more. ‘This permits of the utilisation of a small portion of light radiated from the substance of the image itself. “A large black lens also permits it to be brought nearer to the picture, and this is advantageous, especially with the condensers of the common ordcr, as itacts in condensing the scattered rays from those of this class, enabling also the light to be approached nearer to the condenser. The lens tube should be longer than in the case of its application to photography, for, unlike this, all it is required to cover is the very limited area comprised in a plate threc and a-quarter inches square, minus the vortion uccupied by the mat. For the highest class of objective, it suffices that it be achromatic in the sense different from actinic, for, so long as the visual image ts perfect, it matters not what becomes of the violet or chemical rays, : or what relation they have to the luminous ones. It is in the construction of a lantern objective of short | focus that the skill of the optician is taxed, as it has to cover sharply to the margin with its full aperture, and ; under circumstances in which the slightest inequality in | the definition is instantly detected. ‘Lo acnltivated eye it is extremely unpleasant to sce an image quite sharp in the centre of the disc, and falling off rapidly towards the margin, or by racking in, securing marginal sharpness at the espense of the centre. Of the various forms of objective to be met with in commerce, at any rate for those cf medium short focus, I incline to give preference to that introduced ten: or eleven years ago by J. H. Dallmeyer, judging by the performance of one of this class in my possession, made’ by Newton and Co. In it the elements of the back lens are separated to an extent which would prove hostile to sharpness in the case of one émployed in producing a photographic image in the camera. If photographic lenses are to be employed in ( the lantern, those of the carte-de-cisite (Pctzval form), | that is those corrected for fiatress of ficld, even to the extent of there being slight astigmatism, are advan tageous. One of the most satisfactory short focus objectives I ever used had a back lens 2tin. in diameter, the front lens being |jin. [I gave a very great excess of negative sphcrical aberration to the back lens, and the front was a nearly plano-convex achromatic of short focus. This gave a field which was singularly flat, the definition at the margin quite equalling that in the centre ; but owing to the excess of aberration spoken of, the image did not quite equal in sharpness that obtained by the ordinary carte-de-visite lens with rounder field. Still, spectators scated at a distance of five yards from the screen were unable rcadily to appreciate that the definition was imperfect, for, as you know, cven the crude brush work of the scene-painter seems sharp when viewed from a distance. In objectives of long focus there does not appear to be the same tax on the skill of tae optician. Poor, indeed, must be tho lens of 10in., }2in.,or l4in. focus that will not cover sharply and uniformly a plate Jin. in. dimensions. ‘a: Editorial Table. NammEth.—We have received from the patentee and manufacturer, Mr. A. Gray, 44, Snow-luill, E.C., an apparatus for producing names, numbers on photographs. descriptions, or The outfit is packed in a well-made polished box, and should be welcomed by all wishing to nme their prints in a neat and uniform manner. The desired name is set up in the holder, with the rubber-faced type (and presents tuis style: 44, SNOW-HILL, LONDON,) and dabbed upona pad containing a special ink termed “film:te.” This is then pressed on a sheet of the medium supplied, which causes the name to be imprinted upon it. Bronze powder is then dusted over it to cause greater opacity. he film is then attached to the negative in the position selected, by touching either end of the strip of medium with a little vinegar, and the operation is complete. Each print taken from the negative will have the name in clear and distinct wh'te letters. DEAD Brack.—A bottle of dead black sent by Messrs. Humphries & Co., 268, Upper-street, Islington, has enabled us to do a little touching-up on our lens mounts. We also find it very efficient !or .pplying to wood, or stopping out lantern slides. A sixpenny bottle will cover several square feet. StERN’S SINGLE AND STEREOSCOPIC CAMERA.— A hand-camera which can by one movement be transformed for single (4 plate), or stereoscopic (34 by 6;), negatives should be a desirable instrument. We recently spent a portion of a day making negatives with this apparatus. The plates (12) are conveyed to the exposing plate by the well-known pliable reservoir,