The Optical Magic Lantern Journal (August 1890)

Tho Optical Magio Lantern Journal and Photographic Enlarger. 21 that it means that the picture will be sharper, or better defined, over the entire surface. The reason for the first advantage is that the cone of light that is taken in by the lens will be larger in diameter at the place where it first meets the lens ; and the reason for the second advantage is that the curvature of the lens being less, the picture would be naturally sharper. The ordinary } size tube, that has been in use for years upon lanterns, has a diameter of about one and five-eighths of aninch : I mean the back lens has that diameter ; the focus of this lens is varied from 3in. to 44in., consequently it has been placed at either of these distances, which we call its focal length from the picture. Now the cone of light that is received from the condensing lens upon this back lens has been in length about six inches, depending a great deal in its length upon the focus of the condensing lenses, although not varying more than about 2in.; consequently it follows that when a cone of such length is received from a condensing lens of from 3in. to 4din. in diameter, on an objective lens having a focal length of from 34in. to 4in., it will receive upon its back lens that Cone at a point in its length where it will be larger in diameter than the diameter of the back lens ; consequently all that portion of the cone which is greater in diameter than the lens itself will fall outside of the lens and be lost on the brass work of the mountings. Now if it is necessary for certain kind of work to have a lens of just this focal length of from 3in. to 4tin., that means if you want a picture of a certain size at a certain distance that only this power lens will make, we should have, in order to get all the light that the lamp and condensing lenses will give, a lens larger indiameter—as much larger as the cone of light is larger than the } size lens just spoken of. In fact it should be somewhat larger still. This explains why, with a larger diameter lens, we get more light. Now those lenses that have a long focus are usually made larger indiameter. Recently makers of magic lanterns have been having their short focus lenses made larger in diameter to gain the advantages just spoken of. Increasing the diameter of the lens and maintaining the same curvature will not alter the quality of the lens at all in reference to its flatness of field. This can be only accomplished by reducing the curvature and consequently increasing the length of its focus, and the result of this is reducing its power. We have found that the half size objective will make a better defined picture than the quarter, because the curvature is less, and we have found that the 4-4 tubes will makea still better one for the reason that its curvature is less than that of the 1-2 size. Makers of lanterns have been, I may say, forced to use the objective lenses that have been inthe market suitable for lantern work without regard to their weight or bulk, keeping in mind only the fundamental principle that I have just explained. The result has been that the low power lenses, those that are used at medium-long and long distances, are very bulky and heavy, especially the 4-4 tube. If one will examine, for instance, a lantern that is in operation with a 4-4 tube, and take note of the cone of light that passes into the objective lens, through it and out to make the picture, he can easily discern the cone as it passes through the various lenses, especially if his lenses happen to be dusty. He will notice that that cone of light in diameter is much smaller than the diameter of either of the glasses through which it passes. Now why cannot the lenses be cut down in diameter nearly to that of the cone of light ? This would reduce the bulk considerably and consequently the weight. While upon this subject of reduction of weight and bulk we think also that the mountings are unusually heavy. The front tube, which carries the cap, is much larger in diameter than is at all necessary for lantern work. It could not only be reduced in diameter, but also in depth ; in fact, the whole shape and construction of the tube could be altered to great advantage. It seems to have been merely copied from the camera tube without regard to its utility as a lantern lens. Returning to the position of the objective lens in the cone of light, we would call attention to the fact that the medium-long and long focus lenses necessarily occupy a place in the cone of light, which is further away from the condenser than that occupied by the } size tube, and consequently if the objective lens is not of an extremely long focus it will be nearer the apex of the cone, thus combining the two advantages of a less curvature of glass and a glass whose diameter is much greater than the cone received, and this is why the larger lens gives a much brighter and much flatter field. Those who have used the two extremes, the 1-4 and the 4-4 tubes, will have noticed a peculiarity in focusing their pictures on the screen. With the 1-4:size tube they can quickly focus, because the movement of the rack will be comparatively long; while when they have been using the 4-4 tube the movement of the rack is very short, the result on the screen not being so decided. I do not know whether I can make myself exactly plain to those who have not appreciated this, ibut the moment you touch the rack movement of the 1-4 size tube there is an immediate change upon the screen of the focus of the picture, while with the 4-4 tube the change is not nearly so apparent.— The Exhibitor. :0!—= To Prevent Softening of Negative Films in Hot Weather. Mr. BACHRACH, in a communication to W7/soz's Photographic Magazine, says :—‘* At this season of the year, on extremely hot, sultry days, gelatine films are sometimes difficult to handle in the absence of ice or cold water, and they often become very soft in the developing solution. A perfect cure for this will be found in the addition of about half-an-ounce of a solution of sulphate of magnesium (ofa strength of one ounce to eight ounces of water) to each pint of developer. It is especially useful.with eikonogen, and is perfectly inert, having no deleterious effect whatever. It acts almost as energetically as alum, and has none of the objectionable qualities ofthe latter. With pyro developer I have found the same quantity of a saturated solution of bichloride of mercury to answer the same purpose, but with eikonogen I prefer the sulphate of magnesium. It leaves the film as firm as in cold weather, and ice or cold water can be entirely dispensed with. I still adhere to eikonogen for portraiture.”