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444 THE PHOTOGRAPHIC NEWS. [July 13, 1883. remain steady for a few seconds more, and an exposure again made for a very brief period, by which the transparency is impressed upon the sensitive plate, the sitter acting as a mask. Upon developing the negative, the portrait of the sitter will be seen backed by the scene represented in the transparency, and which is now a faint negative. The strongest effects are pro duced when the principal exposure, that of the portrait, is made with a drab background behind the sitter, for which one much lighter in colour is substituted during the exposure with the transparency. A convenient holder for the transparency consists , of a light wooden frame, hinged at the bottom and at a little distance in front of the dark slide containing the sensitive plate. This lies on the bottom of the camera while the sitter is being taken, but is brought into an erect position as soon as the white background has been substituted for the dark one. The second exposure should be quite momentary, as the landscape scene in the finished negative ought to be somewhat faint, and not absolutely sharp. Sharpness of this scene depends upon the distance at which the transparency is held from the sensitive plate. Any ingenious amateur can easily make for himself a trans parency exposing frame of the kind just described. THE USE OF THE DIAPHRAGM. BY CHABLES EERMAXN.* INTRODUCING to you this very interesting and instructive subject for the purpose of discussion, I can only call to your attention the careful observance of axioms long established in photographic optics. I do not attempt to tell you anything new, but merely quote from authors well known to you. “ Hardwich,” American edition, 1855, pp. 55 and 56, says : Both chromatic and spherical aberration reside principally in the outside or circumferential portion of lenses, to remedy which it appears necessary to cut off the outside, and use only the central part of the lens. Now, this is precisely what is effected by a stop, which is a movable diaphragm with a circular aperture, intended to be placed in front of the lens in order to intercept that portion of the light which would otherwise have fallen upon the edge of the lens. The employment of a stop is always attended with the dis advantage of cutting off a very large portion of the light, and so of diminishing the brilliancy of the image. It is more resorted to, therefore, in cases where a rapid action is desired ; but when time of exposure is no object, a better result in very many ways is secured by the use of a stop. Even supposing the lens to be so carefully ground—which, however, it rarely is—that the spherical aberration is perfectly corrected, the definition of sharpness of outline of the image is improved. The various rays proceeding from the object being confined to the central portion of the lens, interfere with and confuse each other less than they would otherwise do. They are made to fall upon the base at a higher and more equal angle, and therefore the distinctness of focus and flatness of field are proportionally improved. From the same cause, it also happens that when a stop of comparatively small diameter is used, a variety of objects, situ ated at different distances from the lens, are all in focus at the same time ; whereas, with the full aperture of the lens, objects near at hand cannot be rendered distinct at the same time with distant objects, or vice versa. Mr. Taylor, in his revised (ninth) edition of the same work, says, further : Lenses employed with the full aperture do not render near and remote objects sharp upon the ground glass of the camera at the same time. If the foreground is in focus, the lens must be thrown inwards to make the distance clear. This is a necessary consequence of the focal plane of any object varying with the position of the object in regard to the lens. The action of a diaphragm or stop is seen to great advantage in remedying the defect complained of. Rays of light radiating in all directions from a luminous point, and falling on the entire surface of the lens, will not produce an equally sharp image upon the ground glass. Through the action of the diaphragm, set before the lens, a portion of the rays which would impinge upon the outside of the glass is intercepted. The focus in each case is at the point where the rays cross each other after refraction. If the ground glass is advanced nearer to the lens with full aper- ’ Read before the Association of Operative Photographers of New York. t ure, or withdrawn from it, even in the least degree, the image will immediately be out of focus ; but if the rays falling upon the margin of the lens are intercepted by a stop, they will run so nearly parallel, that the effect of a slight change of position will not be perceptible. The refracted pencil is, as it were, sharpened and drawn out, so that the focus has considerable depth, and is no longer confined to a single plane. The focal variation for near and distant objects is much more considerable when lenses of very long focus are employed, and it becomes less evident with lenses of short focus. Of all photographic objectives, the single achromatic or land scape lens is the most simple. Similar to the plano-convex is the meniscus. If the meniscus form is very pronounced, a smaller diaphragm is necessary to correct the aberration than with an objective more nearly approaching the plano-convex, in which case, the larger the aperture employed, the more rapid the action secured. In landscape lenses the stop is generally placed before it, at a distance of about one-fifth of its focal distance. This distance varies with differently-constructed lenses, as, for instance, in the Dallmeyer landscape, composed of three menisci, the stop is set at a distance of one-tenth of its focus. A defect of the landscape lens is its inability to produce rectilinearity. Focussing a square with a single lens, the stop placed before it, the image appears on the ground glass with the marginal lines curved outwards ; while, if the stop is fixed back of the lens, a bending of the lines towards the centre is observed. The cause of this distortion is a deflection of the marginal rays. By placing a stop between two lenses of similar construction, the errors are equalised, and a correctly-drawn picture is the result. This fact has led to the construction of objectives with a central top. The photographic objectives of modern construction have the stop placed between the two lenses. Its exact position towards the lenses differs in different instruments. The apertures in diaphragms are systematically arranged—their diameters, con dition, the time of exposure, and the proportions between them being based upon mathematical calculation. DUST IN THE EYES.—A SCREW LOOSE, OR WHAT ? BY A. L. HENDERSON.* A special correspondent of the Photographic Times and Ameri can Photographer, of the 15th June, gives an account of a dry plate manufactory. Statements therein contained appear so startling that it is as well, before believing all we read, to take penciland paper and see how the statements agree. Sixty dozen plates per hour are coated by each machine, and 400 ounces of silver nitrate used per week. Now we will pre sume, for argument’s sake, that only half-plates are coated, and we find that to use this quantity of silver converted into bro mide, allowing four grains for each plate, that this would coat 43,750 plates. Now, to coat these plates at the rate of sixty dozen per hour, would require only one coater, working sixty hours, forty-five minutes, fifty seconds. Further comment is superfluous. Suffice it to say that this statement reminds me very much of the story of the young American who, being asked where liars went, replied that they went down West to write for the papers ; but it seems that, having now done with the West, they are making their way Eastward. B Zictionary of Dhotograph. ABBREVIATIONS.—Such shortened terms as “pyro” instead of pyrogallic acid or pyrogallol, and “ hypo " instead of sodium hyposulphite—or, rather, thio-sulphate—are thoroughly estab lished in photographic literature ; but these abbreviations are not likely to mislead the general reader. Many speakers and writers ordinarily refer to silver nitrate as “ silver,” and potassium bro mide simply as “ bromide.” Abbreviations of units of weight and measure are so well understood that a list of them would be superfluous, but it may be mentioned that gr. in most continental writings stands for gramme. The English usually abbreviate gramme to gm., and grain to gr. • Read before the London and Provincial Photographic Association.