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320 THE PHOTOGRAPHIC NEWS. [JULY 4,1862. sideration will show that this does not afford the kind of ex posure required for the purpose. On referring to fig. 1, where AB represents an object in front of a double combination lens used with the full aper ture, ab the image formed in the prepared plate, and S the Fig. 1. shutter placed just in front of the lens, it will be seen that as soon as the shutter begins to rise, and uncovers the edge of the lens, as represented in the figure at D, part of the rays forming the pencils emanating both from A and B, will enter the lens and be refracted to a focus at a and b respec tively. The same will be the case with the pencils of rays emanating from all intermediate parts of A B; so that as soon as soon as the shutter begins to rise an image is formed over the entire plate, very feeble at first, and con fused by the diffraction suffered by the rays in passing through the narrow opening, but gradually getting brighter until the lens is entirely uncovered, when the illumination leaches the maximum. This, however, is no sooner attained than the shutter must again begin to descend, gradually cutting off the light; and on its approaching the edge of the lens, again causing indistinctincss in the image by diffraction. In this way, instead of the requisite amount of light being admitted in the least possible time, a prolonged, partial exposure is obtained. But it is also well known that the effect of an exposure of this kind upon a sensitive plate is different from that produced by the same amount of light admitted in a much shorter time ; in the latter case the tendency is to produce an intense and vigorous negative, but in the former a flat and feeble one, and want of inten sity is one of the chief difficulties to be contended with in the prosecution of instantaneous photography. These causes, it is true, can only exert an influence for a very short time, but as that is all the time allowed for the formation of the latent image, it becomes a matter of considerable importance to attend to them, the more so as a plate adapted for this kind of work may be entirely spoiled by that which would produce no effect on a less sensitive one. If the shutter be removed to some distance apart of the lens, as indicated at S', in the figure, it is evident that an excess of exposure may be given to the foreground of a landscape; but the same objection applies, as before, for rapid work, since the field of view will be longer in being entirely cleared. These remari । apply principally to the use of double combination le® 5 । with the stop in a central position, such being now unite sally used for quick work. I With a view to the obviating of these disadvantages, to* writer contrived the new shutter, the properties of whid he is now about to describe. This consists essentially“ two opaque plates, or membranes (both being used) providd with rectangular openings at a distance from each otW equal to that between the lenses. These plates an placed that each covers up one-half of the lens, and t exposure is afforded by their moving simultaneously" opposite directions in a plane at right angles to the of the lenses, which are thus uncovered from the ce"t outwards, and again covered up from the edges toward® centre by the plates still moving onward. In this way * principal part of the lens remains uncovered from first last, thereby affording nearly the maximum illumination the image during the whole time of exposure. . I The difference between the amount of light admitted il the camera during an instantaneous exposure by this oI tri vance, and that admitted by the ordinary one, will beodl more aparent on comparing the amount which will Pl through one-half of a lens when uncovered by a plate slid I from the centre towards the edge, and from the edge towdP I the centre respectively. *1 Let A, B, I), E, (fig. 2) represent a lens, and let the 2a A, B, I), C, be supposed to be divided into any numbera parts of equal breadth, and let the areas of these be daddy by the letters a, b, c,...x, y, z. These areas will evidet Fig. 2. decrease rapidly in size towards B. In the first case, when the plate is drawn from C to B, the largest s a, b, c will remain entirely uncovered during the times pied by the plate in passing over all the other areas; boa small areas, x, y, z, near the edge, will scarcely 1 uncovered any length of time. On the other hand, .< the plate moves from B to C, the small areas at the edgs first uncovered, and the largest and most important near the centre scarcely remain uncovered for any all. Now, as the amount of light admitted in any caSjst be proportional to the area of the lens exposed, and 010 the time it remains exposed, it will be proportional joe product of these. If the respective amounts of light adt • into the camera in these two cases be calculated Wa” application of mathematical formulae, they will be foto? be very nearly in the proportion of 3 to 2 ; and since, c* the most perfect lenses, the margin is of less value I equal area at the centre, we shall not be guilty 0 exaggeration, at least in adhering to these numbers, nddl In uncovering the lens, A, B, I), E, by the newsng above-described, one limb will descend from C toBduntio i] 1 time (supposing the exposure to be of the same dura t0 (, both cases) in which the single shutter rises from "ill, and the amount of light admitted by it in that tim e represented by 3, compared with that admitted by tilb shutter as 2 ; but during the same time the other 1is‘ moved up to E, and admitted the same amoun throng single other' thrice entire! single coveret C to 1 in that times during other 1 the or during surface close p amoun exposu Whe tageon also; I genera of the distant Dary f taken tare, w distant Befo ta a c front < longiti plate, ; upon time, of this heons Suppoi the pl; Incove and if to fill Parts v which image. 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