113. A brief description of the various forms, and an idea of what they are intended accomplish, will therefore be all that it is neceaaary to give here, so often have the lines and principles and forms of the various grades and kinds of lenses been given in more pretentions work We have then: First, the "single"combination, for use in making pictures of objects without motion, when the time of exposure is not necessarily a consideration. They are usually of longer focus than the double lenses, and therefore a much larger picture may be obtained with one of a given diameter than with one of the double form. Second, the "double" combination, which works with greater rapidity than the single, for the following excellent reasons: Behind the first pair of lenses a second pair is placed, which so intercepts the rays refracted from them as to cause them to focus about one-half the distance of the other, thus making them work with far greater rapidity. Third, we have the triplet'" combination, wherein a third pair of lenses is added, or such a combina-tion as to diminish the distortion or obliquity of the rays which pass from the front pair, thus making it the most desirable form for landscape work. 114. These three combinations give us, then, for the two great classes like the outer ones - the case is very different, fur they may ho variously transposed, and thus rendered incapable of producing good pictures. There is a risk, also, of breaking one of the glasses of the back lens in screwing it in, unless it be put together in the proper manner. Many good lenses have been condemned as hopelessly bad through being thus transposed. - Jabez Hughes.
113. In a portrait combination there are four lenses in all, the so - called front and back lenses being really each formed of a pair. The front ones are always cemented together, and may thus be easily taken for one lens; the back pair are distinct, and are usually separated from each other by a narrow ring. To place them in their proper positions, proceed as follows: Take the front lens, the pair cemented together, and observe that one surface is considerably curved, and the other almost flat; place the lens in its cell, so that when screwed into the tube the curved side will be to the sitter. The two glasses forming the back lens are very unlike each other; one is thick at the centre and thin at the edge, the other thick at the edge and thin at the centre; put the thin-edged one first into the cell, resting on the least curved side; next put in the ring, and then the thick - edged glass, concave side towards the other lens; fix them in their places with the part provided, and screw the cell in its place. - Jabez Hughes.
114. With many portrait lenses there is an arrangement whereby the front lens may be used as a landscape lens; to use it for this purpose proceed as follows: Unscrew the hack lens and lay it aside altogether, as it is only required in the double combination; then remove.
of work, portraiture and landscape, the necessary rapidity and correctness of figure or freedom from distortion, with all the roundness, depth, detail, and sharpness of outline which we see in nature. As to dimension, that is governed by the length of focus of the lens used. Given a lens of a certain diameter, the size of the picture will increase with the local length of the lens, and vice versa. It is also increased or diminished according to the convergence or divergence of the lens.
115. There are four other points which must be borne in mind when operating with lenses. 1. If you place the lens too near the object to be photographed, or fail to use the proper diaphragm, so as to stop the action of the lateral rays, you lose definition, obtain an insipid image, and, for your pains, have a distorted picture. 2. If you use too large a diaphragm, or increase the length of focus in the camera, you lose a quality in your pictures known as depth of focus. 3. The illumination of your subject, the size of the diaphragm used, and the focal length of your lens, all govern the time of exposure. 4. Some lenses are so carelessly " corrected for color " as to cause the rays to overlap one another, and thus destroy the quality of definition.
the brass hood before the front lens; next unscrew the front lens and rescrew it in the place where the back lens was. In doing this the flat surface will be presented to the object. The lens tube may be now put on the camera, and the central stops will be in their proper place for use. As the focus of the front lens, when thus used singly, is much longer than when used in combination with the back lens, the picture it will yield is proportionally larger, but a much smaller stop must be employed. - Jabez Hughes.
115. Now, knowing all the defects of lenses, and the different modes of correcting the same, let us look back to that primitive instrument - the pinhole camera. The pinhole camera is free, from all the errors, as spherical and chromatic aberrations, distortion, curvature of field, astigmation, and the only objection against it is the extremely small aperture. What an amount of speculation and hard labor of the most eminent men was necessary to furnish a substitute, equally free from errors, having a larger aperture, giving a brighter image. And, even now, none of the aberrations can be completely corrected, and the best that can be done, and that for a limited aperture only, is to reduce the errors so far as to diminish their extension, so as to make them appear to our eye at a smaller angle than the eye is able to distinguish. In lenses used as objectives, where the image is magnified by high eye-pieces, even that is extremely difficult, as the errors are also magnified. Our most celebrated opticians, such as Fraunhofer, never attempted to give a telescope objective a larger aperture than the focus divided by ten, except in very small pocket telescopes. And his larger telescope, the one he made for the Dorpat Observatory, and which he considered his best objective, has a focus of one hundred and sixty inches, while the aperture is only one hundred and eight lines, that is one-seventeenth of the focal length, and its highest magnification is seven hundred and twenty times. The larger telescopes of Dollond are nearly twice as long. The same artist, Fraunhofer, took precaution to warn young opticians