The disks were thick enough to make two lenses each so we sawed them into two as illustrated in Fig. 3. A is an old polishing head upon which was mounted a pulley at one end and a copper disk, B, at the other, the disk being held between large washers. C is a cast iron box fastened to an arm, D, hinged at E and kept pressed against the copper disk by a cord passing over two pulleys on the ceiling. This made a most excellent automatic feed. The glass to be split was fastened to a block of pine with pitch and the wood held in the iron box, C, with wedges. Number 40 Carborundum was used with plenty of water and the glass was cut through faster than a power hack saw would cut through steel. The glass should be cut half way through and then reversed so that the final break will come in the middle and thus prevent the edges from spawling off. The chief defect of this machine was the way it scattered emery.
The disks are now ready for the grinding which is done on the machine on the right of Fig. 3, which consists simply of a vertical spindle run by a quarter twist belt from the counter shaft against the wall. The end of this spindle is tapered at the upper end to receive the grinding tool or laps, shown on the table in Fig. 5 which also shows the spindle raised so that the grinding lap is seen above the tin box, C, which surrounds the spindle to catch the abrasive that is thrown off in grinding. The glass is first smoothed down on a flat lap until it is of equal thickness at all points as measured by a micrometer when it is ready to be ground to the proper curves. For this purpose the spherical laps, shown in Fig. 5, are turned in the special machine illustrated in Fig. 4. The compound rest of an old Seller’s lathe was removed and in its place, on the cross slide of the carriage, was mounted the sphere turning rest. This consists of a base, A, in which the slide, B, is so mounted that it can be rotated about the center, C, by turning the milled head, D, which carries a worm at the opposite end. E is the tool post with the cutting tool T and L the lap to be turned. A hole was drilled at C into which was fitted a round piece of steel the upper end being pointed and then half cut away like a center reamer. This was used in finding the zero; the rod, pointed end up, was placed in the hole at C and the cutting tool adjusted against the flattened side. The zero position is then determined by measuring, with an inside micrometer, the distance from the tool post to a stop placed at the end of the slide B. By adding to or subtracting from the zero reading of the micrometer the length of the radius of the grinding lap, the tool post may be set to the proper position for either a convex or a concave surface. This, however, is only approximate, for these laps must be made with the highest possible accuracy. After sufficient cuts have been taken to give a spherical surface, the radius is carefully measured with a special spherometer and the error in the radius corrected by changing the position of the cutting tool by an amount calculated from the readings of the spherometer. This spherometer we were compelled to build as we could find none of sufficient accuracy on the market and it is described in a note at the end of this article.
