Materials, Machines and machines in general use (with the exception of those referred to later for the mass production of high quality, lenses) are very simple and pretend to no accuracy of construction. The accuracy of the surface to be produced is very high. A piece of plate glass of, say 6 in. square may have inequalities of in., and from a number of speci mens a piece with errors no more than in. may be selected. The surfaces of good spectacle lenses depart from true spheres by amounts of the order of in., while the optical work in good binoculars has rarely errors of as much as in. ; and, finally, the best optical work of the few firms of highest repute may depart from the ideal aimed at by less than 1 X in. To obtain such precision by mechanical guidance of the tool would be a hopeless task, even were the mechanism perfectly rigid, the polishers free from wear or flow, and the films of polish ing substances of invariable thickness. None of these conditions are complied with, and the optician must rely for the production of accurate polished surfaces on quite other principles than those used by the engineering machinist. He is satisfied, then, with a machine which will move his polisher to and fro over the surface of his lens or block of lenses—from say 6 to 25o times a minute, according to the size of the work. Simultaneously the tool, or lens, whichever is undermost, rotates on a vertical spindle, the uppermost element being allowed to rotate also. The pitch (or other base for the polishing material) is not a solid, but a liquid which, though rigidly resisting distortion in passing over any small inequality of surface, yet, by a slow accommodation to the surface (spherical in the main) over which it passes, retains a spherical shape whose radius of curvature is always that of the surface it is polishing.
The Trueing Tools are usually made of cast iron, turned on the working surface to the desired radius of curvature. There are always two tools of the same radius, convex and concave, and after turning they are ground together with fine emery of the grade known as "trueing" until they fit exactly. The polisher holders (convex and concave) are similar to the trueing tools, but should be of radius of curvature greater or less than that of the trueing tool (according as the latter is convex or concave) by such an amount that the polisher is of a uniform thickness. If a number of lenses are to be polished together they are held on a similar tool by blobs of cement known as mallets.
Polishers are usually of pitch, although felt is sometimes used for the commoner work and various wax mixtures are also used. For pitch polishers wood pitch (e.g., Swedish) may be filtered when hot through gauze and then boiled until, at the temperature at which it is to be used, it can be indented readily but not deeply by pressure of the thumbnail. Although this rough test is often the only one applied, it is the better practice to use a mechanical device, e.g., a weighted cone or ball, applied for a definite time and to measure the indentation produced by it. Since the viscosity of pitch is halved for each C rise of temperature, it is sary to vary the pitch according to the temperature of the work shop, for if the pitch is too hard it causes scratches, while if too soft it soon loses its truly spherical shape, and the lenses get a bad figure. A typical mixture for mallets consists of two parts of pitch to three of red ochre; instead of the red ochre, wood ash may be used.
Grinding and Polishing roughing, the fastest material is carborundum ; for trueing and smoothing various forms of aluminium oxide are used. The impure forms known as emery have been used for grinding from time immemorial, and up till the end of the 19th century the emery from the Isle of Naxos had formed the principal source of supply for the optical industry. Purer and better natural forms known as corundum are found in the United States, Canada, Madagascar and elsewhere, while the artificial forms sold under various names (aloxite, alun dum, etc.), are better still. All those mentioned require grinding
and grading by elutriation; a very fine and uniformly graded material being of the highest importance if speedy polishing is to be accomplished. The following table shows the average sizes of grain suitable for optical work: The finest of these materials are very expensive to produce. In 1923 a further form of A1203 (Sira abrasive) of great purity. hardness and uniformity of grading became available. Owing to its sharp crystal edges, and to the fact that they maintain their quick-cutting properties, grinding with Sira abrasive is a rapid process. This abrasive is not so fine in grain as some of the grades of emery mentioned, but on account of its uniformity and because of the shape of its grains, it gives a ground surface with shallow pits of even depth which can be more quickly polished than the ground surface obtained with the usual series of pro gressively finer emeries. There are two materials in general use for polishing lenses. The first, jewellers' rouge, is a fine red iron oxide prepared by calcination of ferrous sulphate. The second is putty powder (tin oxide), which is often used with polishers of the wax class, and particularly for polishing soft materials. Other sub stances are of occasional use for special purposes, such as diaman tine and chromium oxide. All these are used wet.
The principal subsidiary processes are the and of the lenses, and the of achromatic lenses. The process of and has been briefly described in the introduc tion. Machines are now made where the process becomes semi automatic. For instance, one lens is mounted and adjusted on one spindle while a second lens is by a diamond lap or corundum wheel, on the second spindle, the two spindles mounted so that they can be by about a common centre. For the of achromatic objectives, Canada balsam is still used, and is hardened around the rim of the objective by in a warm oven.