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306 THE PHOTOGRAPHIC NEWS. Mat 18, 1883. actuate his shutter by a single elastic band, one end being attached to a central knob fixed to the disc, and the other end to a pin at D ; but he found that the one-sided drag A is a plain wooden shutter with a slat or groove, BB;Cisa disc, working on a centre, F, and fixed in it is a pin, G, working in the slot, B B; Disan india rubber band fixed over a pin at D, and over another at F. offered a considerable hindrance to the free motion of the disc ; but when another band was attached to an opposite point of the knob and to a pin near E, all worked easily and freely. An ordinary stop and release working on the edge of the disc serves for governing the shutter, the speed being regulated by the tension of the bands. Mr. Cowan’s shutter should be fitted in the camera immediately in front of the dark slide, and the projecting part (con taining the disc) should be directed downwards, as under these circumstances the slight inequality of exposure over the various parts of the plate tends to give a somewhat more prolonged exposure to the foreground than to the sky. Mr. Cowan’s shutter can easily be made so as to cover an area no larger than that of the dark slide when the flap is drawn, and in this respect offers many advan tages over the simple drop shutter, which would, under ordinary circumstances, cover nearly double the area of the new shutter. WIDE-ANGLE LENSES. What is the property which constitutes a lens a “wide- angle” one? This appears a simple question. “Why, the property of including a wide-angle! ” nine out of ten will say ; and this is certainly true. Yet, on enquiry, we find that many have not a thorough understanding of the matter, but imagine that a wide-angle lens will, on a given plate, take in more subject than will a narrow angle lens of the same focal length. This is not correct, for it must be understood that, barring the slight real distortion given by single lenses, any lens gives, as far as size and outline of the image are concerned, exactly the same effect as any other lens of the same focal length. What constitutes a lens a wide-angle one is its property of bringing to focus in the same plane as the central pencils focus at, oblique pencils which, with a narrow angle lens, would either focus in a plane farther forward, or would be actually out off by the lens mount, or tube In fact, the two qualities necessary for a wide-angle lens are, first, that the general form of the instrument should be such that marginal rays shall not be cut off by the lens mount; and second, that there shall be flatness of field. If we leave out of thequestion that of pencils of light being actually stopped'by the brass-work of the instru ment, a lens of flat field and a wide-angle lens mean the same thing. A lens is only “ wide-angle ” in that it will allow, with any given focus, the use of a larger plate than will a narrow-angle one, or, what follows from this, in that it allows with a given plate the use of a shorter focus than will a narrow-angle lens, and gives definition to the corners of the plate. There is, however, not wisting about of the pencils of light to take in a great amount of subject, as many appear to suppose. We often hear of the distortion produced by the use of wide-angle lenses. Most are aware, however, that this dis tortion is not real, and that, if the picture which exhibits it be looked at from a certain point, it entirely disappears, the fact being, however, that this point is one from which no one would ever think of looking at the picture, inasmuch as it is so near that the whole subject cannot be seen at one time. Still, if we are able to focus our eye at all to so short a distance, we shall find that by looking at a picture taken by a wide-angle lens, from a point corresponding to the equivalent focus of the lens with which it was taken, we cause the appearance of distortion to vanish from that small central portion of the picture which our angle of vision does include. To prove that there is no real distortion, we need only photograph any object all in one plane at right angles to the axis of the lens. The best thing of all is a sheet of paper stretched on a drawing board, and having straight intersecting lines drawn on it. However wide an angle be included in copying this, the lines will be brought out straight, and will retain the same proportionate distance from each other. We are, of course, at present leaving out of consideration the distortion peculiar to single lenses. There is one appearance produced by wide-angle lenses which it is at first difficult to believe is not actual distortion. If a set of spheres—croquet balls, or anything of the kind will do well—be placed on a table in a straight line, so that, when photographed with a wide-angle lens the extreme ends of the row will come on the extreme ends of the plate, a photograph of them will show a most peculiar appearance. The central balls will be represented by circles, but the extreme end ones will be shown as ellipses, having their major diameter passing through the centre of the plate, or, more strictly speaking, through the axis of the lens. Cer tainly this would appear to be a case of very genuine distor tion ; still, it is not. If, in looking at the photograph of the row of spheres, the eye be placed at the point indicated before—that is, atone corresponding to the equivalent focus of the lens used—the image on the retina produced by the oval representation of the balls will be the same as that produced by the balls themselves. It is only when we move our eye farther away that a false effect is produced. The same apparent distortion will be seen in a photograph of a row of columns or cylindrical bodies, as in that of the spheres. Those near the end of the row will be represented as thicker than those near the centre. Although, as we say, these effects cannot be classed as true distortion, they practically militate against a photograph as a picture quite as much as if they could, this being, as we explained, because it is impossible to look at the whole picture from a point sufficiently near. Another defect exhibited by wide angle lenses is their property of giving much less light to the edge of the plate than to the centre. This falling oft of light increases in a very rapid ratio, and produces a very disagreeable effect when it exceeds a ceitain amount. It is quite true that a slight falling off of light towards the edges of a picture produces an artistic effect, but with an angle anything in excess of 60°, this falling off becomes too great. It is the only thing which makes it undesirable to use a wide-angle in copying. In this case, the object to be photographed being all in one plane, there is no chance of distortion, either real or apparent, making itself evident. As far as form is concerned, a precisely similar image is given by a wide- angle and a narrow angle, that is by a short focus or a long focus lens. In the former case, however, there will be a most noticeable falling oft of light at the edges of the plate, in the latter not,