THE GRINDING AND POLISHING OP LENSES.
The piece of glass of which a lens is to be made is cut out of a plate of sufficient thickness, and this is brought roughly into shape by means of pincers, a grind-stone, and art old mould. A pair of guages and moulds are require,d for each surface of the lens. The pair of guages are made by striking the required. curve on a brass plate, and cutting through the line very accurately ; one of the guag,es is therefore convex, the other concave.
Two moulds are then made of brass or iron, and turned in a lathe, to fit the guages. One of these moulds is therefore hollow, the other convex. To ensure accuracy they are then ground one against the other, with some fine emery between them.
The piece of glass to be ground is now attached to a holdfast by a few drops of melted pitch. The holdfast is a round brass plate a little smaller than the lens. The mould is then applied to the upper surface, together with some coarse emery and water, and the grinding is ae,complished by hand, by turning the mould continuously round upon the glass, giving a cross rub now and then, using gentle pres sure, and taking care that the edge of the mould never goes beyond the centre of the glass. When the glass has assumed the shape of the mould, and touches it in every part, the coarse emery is washed off and finer emery substituted for it. The grinding is then con tinued until the surface of the lens assumes a uniform dead appear ance, and all scratches are removed. During this process the mould which is in use is frequently ground against the other, in order to prevent any change of form that might occur in it. Having pro
ceeded thus far, finely powdered pumice stone is substituted for the fine emery, and after a little more grinding the stuface is ready for polishing. The other surface of the lens is then treated in the same way.
It now remains to polish the surfaces.
This is done with moulds of pitch ; and fine colcothar is used instead of emery or pumice powder. (Colcothar is the re,d peroxide of iron, called " Rouge," or jeweller's polishing powder ; q. v.) The polishing moulds are made thus : equal parts of pitch and resin are melted together and poure,d into the metal mould (which is previously heated), to the depth of about a quarter of an inch ; the cold mould is then laid upon the melted pitch, and when it has set the whole is thrown into water and suddenly cooled. The cold mould is then letached from the pitch.
As the polishing proceeds the pitch becomes hot with the friction, and sticks to the glass, so that the labour of polishing increases towards the end of the operation, which must not be interrupted until finished. A drop or two of water is added to the coloothar at first, but no fresh coloothar or water are afterwards added. If the pitch becomes too adhesive it must be breathed upon.
Lenses are sometimes ground by machinery, but hand grinding is thought the best for fine work.
An account of the kind of glass used for photographic lenses, and the defects to which it is subject, is given in the article " Op tical Glass ;" q. v.