The 1 in. rod, at its lower end slips loosely inside a 1 in. gaspipe (internal diam. 13-16 is.) 3 ft. long. When set at the right length the thumb-screw shown, clamps rod and pipe. Through the lower end of the 1 in. pipe two 1 in. round rods are passed at right angles and firmly secured. The outside 3 in. at both ends of each rod is turned down at right angles, flattened at the tips and drilled with small horizontal holes, H H H H near the extreme ends.
Some care should be taken that H II II 11 lie in a plane at right angles to the pipe, and form a square of which the pipe is the center, each hole being, say, 10 in. from this center. Through H. H H H threaded hooks are loosely passed, the hooks being on the inside, and each threaded outside carrying a thumb nut. These hooks are connected with four short hooks screwed into the top of the polisher (near to and at a uniform distance from its edge and 90° apart) by four helical springs or rubber bands all of the same length and stiffness. Each connection may be made part link or light chain and part spring, to allow for varying sizes of polishers.
The top of the 3/8 in. cross-rods may be Used to support a sheet of ordinary window-glass, 2 ft. in diameter, with 11/8 in. hole bored or ground in the center. The space between the pipe and the glass is covered by a cloth washer, weighted with an iron washer to keep it in close contact with the glass. The object of this is to protect the face of the mirror from dirt. The movements of the polisher may be watched through the glass. For the same reason the bottom of the pipe is plugged.
Vertically under the center of the hole in P the center of a square optician's post is located. It may be made of several pieces of seasoned wood, surfaced on all four sides, well glued, and spiked together. It must be firmly fastened to the floor, and the upper surface, which should be the end grain, must be carefully dressed plane and level The whole is then thoroughly painted. This supports a cylinder of wood about 3 in. high, and of the same diameter as the mirror, carefully turned and facedon both ends. This, too, should be well painted. The center is to be plumbed under the center of the round hold in P, and the surface levelled.
This supports two thicknesses of new Brussels carpet cut to the diameter of the mirror. They are covered with a projecting sheet of rubber, or thin oilcloth, to prevent soiling the carpet with drip. Every care must be taken to prevent the drip from working in between the mirror and the cloth, particularly at irregular intervals around the outside, for this would be fatal to the production of a good surface towards the edge. Indeed, with this method of polishing it might be advisable to make the wooden support somewhat smaller than the mirror. This would conduce to cleanliness, and also give the edge a chance to yield a little when long strokes are used.
On the cloth rests the carefully centered mirror, closely, but not tightly, held in place by four adjustable wooden clips, C, C, C, C. These clips als0 serve to support, when used, the outside wooden or metal hoop indicated by the dotted circle in the plan, the use of which is to guide the eye in making long strokes, and also to prevent a careless overstroke. The mirror should be of good homogeneous glass, not rolled, about 1/8 in. diameter in thickness; it should be ground approximately circular, witli the back mechanically flat. The face should be rough-ground to closely approximate the desired focus, and so that the edge will caliper to the same thickness all around. If desired the rough-grinding may be done with this apparatus by taking off the hooks and springs, and connecting the pins in the back of a heavy lead-grinder, of half the diameter of the mirror, directly with the cross-arms. This may be done with wire or stout twine.
The fine-grinding should certainly be done with the apparatus full-rigged. The tool for fine-grinding the 12 in. mirrors (8 ft. solar focus) was of the best, close-grained cast iron, 6 1/4 in. diameter and 3/4 in. thick, finished size. Both faces and the edge were well cut away in order to get rid of the skin. One face was closely turned and scraped to templet, then put in a planer and grooved in squares in the usual manner with a round nosed tool. The edges and corners of the squares were rounded with a file to prevent crumb-bling and scratching.
Polishers may be made of the same material and in the same way, but without the grooving. They should be thinner and ribbed on the back along two diameters at right angles to each other, Into which are screwed the holding-hooks for the springs. If, however, the polishers be of wood, the spring holding-hooks on the back should be made of two strips of flat iron, turned up and over 180° at the ends. These should be screwed to the back of the polisher and at right angles to each other. This precaution is to transmit the tension of the springs across the polisher without straining it locally. If desired, polishers may be weighted, also the grinder. In using this arrangement, Dr. Draper's directions for hand-polishing were generally followed.
If the springs are slack, the weight of the polisher governs entirely. By shortening the length of the adjustable swinging-bar, and increasing the tension on the springs, the tendency is to lift towards the edge of the mirror, provided the length of the bar is materially less than the radius of curvature of the mirror. This setting has a tendency, with straight diametrical strokes, to parabolise a spherical mirror, at the same time forming a good edge. The spherical figure may, however, be maintained or restored by making the strokes with longer bar, and with less tension on springs along chords tangent to the intermediate zones, occasionally zig zagging from one side of the mirror to the other, all the while walking steadily around the post, and turning the polisher by twisting the pipe at the hand hole.
The mirror is occasionally turned a little within the clips with gloved hands, and also the polisher (90° at a time) within the springs. This to insure a uniform effect.
A number of variations in adjustments, not above indicated, may be used for special purposes; but they are hardly necessary. It is to be noted, however, that, with properly-designed hooks, raising H H H H, with reference to the polisher, reduces its pressure on the mirror, and vice versa. Full-sized fine grinders and polishers of any form may be used quite as well as smaller ones, and do their work more quickly.
All sorts of strokes may be used, but straight ones usually suffice. The whole system is flexible, elastic, and under complete control. In short, it is much like a fiddle - it may be tuned up to suit.