The Telescope

by Louis Bell

The next stage of operations is polishing, which transforms the grey translucency of the fine ground lens into the clear and brilliant surface which at last permits rigorous optical tests to be used for the final finish of the lens. This polishing is done generally on the fine grinding machine but with a very different tool and with rouge of the utmost fineness.

The polishing tool is in any case ground true and is then faced with a somewhat yielding material to carry the charge of rouge. Cheap lenses are commonly worked on a cloth polisher, a texture similar to billiard cloth being suitable, or sometimes on paper worked dry.

With care either may produce a fairly good surface, with, however, a tendency to polish out the minute hollows left by grinding rather than to cut a true surface clear down to their bottoms. Hence cloth or paper is likely to leave microscopic inequalities apparently polished, and this may be sufficient to scatter over the field a very perceptible amount of light which should go to forming the image. All first class objectives and mirrors are in fact polished on optician’s pitch. This is not the ordinary pitch of commerce but a substance of various composition, sometimes an asphaltic compound, again on a base of tar, or of resin brought to the right consistency by turpentine.

Whatever the exact composition, the fundamental property is that the material, apparently fairly hard and even brittle when cold, is actually somewhat plastic to continued pressure. Sealing wax has something of this quality, for a stick which may readily be broken will yet bend under its own weight if supported at the ends.

If the fine grinding process has been properly carried out the lens has received its correct form as nearly as gauges and the spherometer can determine it. The next step is to polish the surface as brilliantly and evenly as possible. To this end advantage is taken of the plastic quality already mentioned, that the glass may form its own tool.

The base of the tool may be anything convenient, metal, glass or even wood. Its working surface is made as nearly of the right curvature as practicable and it is then coated with warm pitch to a thickness of an eighth of an inch more or less, either continuously or in squares, and while still slightly warm the tool is placed against the fine ground disc, the exact shape of which it takes.

When cold the pitch surface can easily be cut out into squares or symmetrically pitted with a suitable tool, at once facilitating the distribution of the rouge and water that serves for polishing, and permitting delicate adjustment of the working curvature in a way about to be described.

Fig. 45 shows the squared surface of the tool as it would be used for polishing a plane or very slightly convex or concave surface. Supplied with the thin abrasive paste, it is allowed to settle, cold, into its final contact with the glass, and then the process of polishing by hand or machine is started.

The action of the tool must be uniform to avoid changing the shape of the lens. It can be regulated as it was in the grinding, by varying the length and character of the stroke, but even more delicately by varying the extent of surface covered by the pitch actually working on the glass.