A lens, or rather a doublet lens, is said to be symmetrical when both combinations are precisely alike and possess the same optical properties. In all such combinations the diaphragm is placed midway between the two. Non-symmetrical lenses are those in which one of the combinations is the more powerful in some way or other, in which case the diaphragm is placed at the exact proper distance as calculated by the optician. To enable the amateur to select a lens for his own use, several considerations are necessary, and although he may be to some extent guided by the vendor of the lens, the following may be of some assistance: -
The lens should be absolutely free from striae; these can be detected by placing the eye at the focus of a lens before a strong light, such as gas or a lamp; bubbles can also be seen, but too much stress need not be laid on the presence of one or two bubbles, as when not present in very large numbers they may be disregarded. For what purpose is the lens required? For ordinary landscape work, architectural subjects, interiors, or portraits. For landscape work pure and simple there are few lenses to equal the achromatic single landscape lens, which gives brilliant negatives; and although distortion is present it may be disregarded, as it is practically unnoticeable in small views except by mathematical measurement. Some of these lenses are now made to work aplanatic with an aperture of f/8, and are therefore of nearly the same rapidity as the rapid rectilinear, which is, however the lens par exellence for amateurs, as its use is practically unlimited, especially as some are now made to work at almost as large an aperture as a portrait lens. Few amateurs will require a portrait lens, as they are not only difficult to use properly, but are exceedingly limited in action and also expensive. The question of how much view to include on a plate is another important consideration which should not be lost sight of (see ANGLE, WIDTH OF); for ordinary work it should never exceed 50o to 55o, and 45o is decidedly better, as this is about the angle included by the human eye. If a much greater angle be included, the resulting pictures have a distorted appearance, because it is extremely unlikely that the eye will be placed at the focal length of the lens from the picture. The glass of which lenses are made should be absolutely colourless; this can be tested by laying the lens upon a sheet of white paper and looking down through it. Some of the cheaper lenses are made of glass which is not colourless; and any colour, especially brown or yellow, will make the lens slow.
As stated under the article GLASS, it is absolutely necessary to obtain it perfectly homogeneous, free from striae, colourless and transparent; bubbles, lines, and opaque particles in lenses merely obstructing a certain amount of light, but striae prove imperfect and unequal mixture of the substances composing it, and will therefore give different refractions. The glass is made in the following manner:Crucibles of fire-clay of particular form are raised to a white heat in a furnace, and when the fuel ceases to give off smoke, they are charged with the materials, and the heat is continued for eight or ten hours. The crucible is now raised to a white heat for four hours, and the mixture stirred with a bar of potter's clay. Six times from hour to hour the mixture is stirred. The heat is then reduced, that the bubbles may rise, and again at the end of two hours the heat is raised to make the glass fluid; again stirred for two hours, and the crucible and the openings of the furnace closed and left for eight days to cool. The crucible is taken out and broken, and the glass is removed and divided into pieces. The divided glass is examined and sorted, the finest being retained for astronomical purposes, the second quality for photographic lenses, and the third for ordinary magnifying glasses, the rest being waste, which is added to the next melting. Sometimes the plates are then cast, after being softened by heat into rough moulds of clay or iron coated with sand so as to give them a rough form; but the best opticians prefer grinding, as striae and bubbles are not so liable to be formed, except with condensing lenses in which striae and bubbles are not of so much importance. The rough-shaped glasses have now to be made into perfect lenses, for which purpose extreme care is absolutely necessary, approximate forms being given by grinding with wet sand in concave or convex tools of cast iron. It is in the following operations that the greatest skill and care of the optician are required:The roughly shaped lens is now to be ground with emery in spherical tools of brass or iron, which tools are made either by casting or by rough casting and subsequent work in a lathe. These are given the necessary curves by means of extremely accurate gauges of copper. The roughly fashioned glass or lens is fixed to a plate of brass by means of pitch, and is then worked in the tool with rough emery moistened with water; when the glass is found to touch the tool at all points, finer emery is used, and it is worked a little more, the gauge being now frequently applied to the tool, to see that the radii of curvatures are not altered; then finer kinds of emery still are used, till at last some degree of polish begins to show; fine pumice-stone powder is then substituted for emery, and the polishing is begun. The operation of polishing is really the test of a good optician, as this process may alter the sphericity or the radii of curvature of the lens to such a degree as to completely alter the character of the lens. The lens is fixed on to a block of wood by means of a pitchy cement, and a tool is coated with a resinous mixture, and fine rouge is sprinkled on the tool when cold, and the polishing finished entirely by hand.