111. Perhaps the most important of all the apparatus needed by the photographer in this work is the objective, or lens. It should be selected with the greatest care, though, indeed, our opticians now produce lenses of such exquisite workmanship that one run no risk in making a purchase from any of the well - known makers. The matter of selection, then, lies in the form or kind of lens desired. Any sort of a gun will scare a bird, and any sort of a hammer drive a nail. So will any sort of a lens make a picture. But as excellence ia always to be aimed at and after, the photographer should choose the lens which will produce the kind or class of work he desires, of the best quality. No lens will do all classes of work, any more than one hammer will. No lens to be a rule in optics that if one quality is secured to a high degree, there must neces-sarily be a compensating loss in some other direction. For example, we somewhat a difficult matter to select. Knowing the class of results you want to secure, however, it is a comparatively easy matter to find what, will satisfy you.
111. Light is propagated in a straight line. We cannot see around a corner. If a ray of direct sunlight passes through a small hole of any given shape into a darkened chamber, and we hold a screen near behind the aperture, we observe a bright image of the shape of the hole. If we increase the distance of the screen and the aperture, the image of the hole disappears in the penumbra, and the round Image of the sun takes its place; and, if the hole is small enough, you will see not only the image of the sun, but the image of all the external objects will appear likewise. This is only true when it continues in a medium of the same density, or it enters a medium perpendicular or normal. But, if a ray passes from one medium into another of different density obliquely, its direction is changed - it is refracted. - Joasra Zentmayer.
I have long noticed the peculiar effects produced by rapid - acting lenses, and by extra sensitive chemicals. In the one case, I have remarked that a certain softness, which is tint and most evident peculiarity of negatives made by rapid - acting lenses with a proper exposure, interferes very materially with the clear brilliancy of definition that is most desirable, and, when not too old and hard, is the highest attainable perfecting chemical effect; while with the extra-sensitive chemicals the contrary is the most usual effect; the negative is prone, to hardness and intensity ( when not over-exposed, and then it is fogged), and lack-ing in definition, a very bad quality, and not to be tolerated except under compulsion. It will thus be clearly seen that the rapid-acting lens, when used with full apart smothers the image with its full volume of light; while the lightning collodion does not give 6 (81 may secure depth of focus, rapidity of action, and exquisite definition, but only at the expense of the size of the image. For as you increase the latter, you decrease the other good qualities named.
112. All lenses either scatter (diverge) or concentrate (converge) the rays of light which are transmitted through them. They are, for convenience in grinding, all made with a curved surface or surfaces, and are usually of the following forms: 1, double-convex; 2, plano - convex; 3, concavo - convex; 4, double-concave; 5, plano-concave; 6, meniscus. The effect of the curves of a double-convex lens, say, would be to collect the parallel lays of light which pass through it to a point or focus, while those of the double-concave lens would scatter them, and the other forms more or less converge or diverge, according to their forms. In order, then, to prevent the distortion or spherical aberration which would occur if single lenses only were used, opticians resort to combinations of lenses of various forms together, and to the use of diaphragms or stops. But a few years ago the photographer had but two grades of lenses to choose from - the single combination and the double combination, the former for landscapes and the latter for portraits. Much more attention has been given to their wants of late years by opticians, until now it becomes a properly balanced effect, because of its extreme sensitiveness; the diffused light admitted through the lens (an always appreciable amount) fogs the shadows under a fair exposure, and with a shorter time the high-lights are unduly affected, and harshness results. - E. M.
112. Many of you are aware that in nearly all human eyes there exists an aberration, also called astigmation. Although in its effect similar to the astigmation of lenses, just mentioned, it is of a different character. Nature intends that the curves of the cornea and crystalline lens of the human eye should be spherical; but the exceptions seem to be the rule. The curves of the cornea and crystalline lens of the eye are, in nearly all cases, more or less elliptical, egg-shaped, and consequently have in one meridian a longer focus than in the other. If such an eye brings the image of a line parallel to one meridian to a focus at the retina, the images of lines parallel to all the other meridians do not collect at the retina, especially the one at right angles to the former, and a distorted, blurred image is the result. The advancement of science has lately enabled our oculists to correct this evil by spectacles, of which the glasses are parts of cylinders instead of spheres. - Joseph Zentmayer.
The lenses in a portrait combination are occasionally removed from their cells for the purpose of cleaning. Generally speaking, it is sufficient to unscrew the mounting and wipe with chamois leather the two surfaces exposed. They can then be easily replaced; for the brass fittings are usually so made, that if by mistake the cells are screwed into the wrong places, the hood, or projecting shade, will not go on. The mistake is, therefore, easily detected and corrected. "When, however, the lenses themselves are taken out of their cells - and, except for curiosity, this is rarely required, for the inner surfaces do not become dirty.