The Telescope

by Louis Bell

Fig. 45.—Tool for Flat Surface.

Fig. 46.—Tool for Concave Surface.

This is done by channeling or boring away pitch near the rim or center of the tool as the case may be. Fig. 46 shows a tool which has been thus treated so that the squares are progressively smaller near the periphery. Such a spacing tends to produce a concave surface from a flat tool or to increase the concavity from a curved one. Trimming down the squares towards the centre produces the opposite result.

Broadly, the principle is that the tool cuts the more in the areas where the contact surfaces are the greater. This is not wholly by reason of greater abrading surface, but also because where the contact is greater in area the pitch settles less, from the diminished pressure, thus increasing the effective contact.

Clearly the effect of trimming away is correlated with the form and length of stroke, and the temper of the pitch, and in fact it requires the wisdom of the serpent to combine these various factors so as to produce the perfectly uniform and regular action required in polishing. Now and then, at brief intervals, the operation is stopped to supply rouge and to avoid changing the conditions by the heat of friction. Especially must heating be looked out for in hand polishing of lenses which is often done with the glass uppermost for easier inspection of the work.

Polishing, if the fine grinding has been judiciously done is, for moderate sized surfaces, a matter of only a few hours. It proceeds quite slowly at first while the hills are being ground down and then rather suddenly comes up brilliantly as the polisher reaches the bottoms of the valleys. Large lenses and mirrors may require many days.

Now begins the final and extraordinarily delicate process of figuring. The lens or mirror has its appointed form as nearly as the most precise mechanical methods can tell—say down to one or two hundred-thousandths of an inch. From the optical standpoint the result may be thoroughly bad, for an error of a few millionths of an inch may be serious in the final performance.

The periphery may be by such an amount longer or shorter in radius than it should be, or there may be an intermediate zone that has gone astray. In case of a mirror the original polishing is generally intended to leave a spherical surface which must be converted into a paraboloidal one by a change in curvature totalling only a few hundred-thousandths of an inch and seriously affected by much smaller variations.