J. R. Stephens
In 1878 and 1879 I did considerable work in grinding and polishing glass mirrors of 12 in. aperture, and 8 ft. solar focus. The methods used were essentially those of Dr. Draper, with whom I had correspondence at the time. The apparatus used by me, however, differed materially from Dr. Draper's machine. I will first quote the general method (date 1864) from Dr. Draper, and then give a description of my apparatus.
'* The method of producing reflecting surfaces next to be spoken of is, however, that which has finally been adopted as the best of all, being capable of forming mirrors which are as perfect as can be, and yet only requiring a short time. It is the correction of a surface by local retouches. In the account published by M. Foucault, it appears that he is, in France, the inventor of this improvement.
"The mode of practicing the retouches is as follows (for 15 1/2 in. mirror 150 in. solar focus):Several disks of wood, varying from 8 in. to 1/2 in. in diameter, are to be provided, and covered with pitch or rosin, of the usual hardness, in squares on one side. On the other, a low cylindrical handle is to be fixed. The mirror, having been fined with the succession of emeries before described, is laid face upward on several folds of blanket, arranged upon a circular table screwed to an isolated post, which permits the operator to move completely round it.
"An ordinary barrel has generally supplied the place of the post, the head serving for the circular table, and the rim preventing the mirror sliding off. The other end is fastened to the floor by four cleats.
"The large polisher (8 in.) is first moved over the surface in straight strokes upon every chord, and a moderate pressure is exerted. As soon as the mirror is at all brightened, perhaps in five minutes, the operation is to be suspended, and an examination at the center of curvature made. By carefully turning round the best diameter for support is to be found, and marked with a rat-tail file on the edge, and then the curve of the mirror ascertained. If it is nearly spherical, as will be the case if the grinding has been conducted with care and irregular heating avoided, it is to be replaced on the blanketed support, and the previous action kept up until a fine polish, free from dots like stippling is attained. This stage should occupy three or four hours. Another examination should reveal the same appearance as the preceding. It is next necessary to lengthen the radius of curvature or the edge zones, or, what is much better, shorten that of the center, so as to convert the section curve into a parabola. This is accomplished by straight strokes across every diameter of the face - at first with a 4 in., then with a 6 in., and finally with an 8 in. polisher. Examinations must, however, be made every five or ten minutes, to determine how much lateral departure from a direct diametrical stroke is necessary to render the curve uniform out to the edge. Care must be taken always to warm the polisher, either in front of a fire or over a spirit lamp before using it.
"Perhaps the most striking feature in this operation is, that the mirror presents continually a curve of revolution, and is not diversified with undulations like a ruffle. By walking steadily round the support on the top of which the mirror is placed, there seems to be no tendency for such irregularities to arise. If the correction for spherical aberration should have proceeded too far, and the mirror become hyperbolic, the sphere can be recovered by working a succession of polishers of increasing size on the zone intermediate between the center and edge, causing their centers to pass along every chord that can be described tangent to the zone.
"A most perfect and rapid control can thus be exercised over a surface, and a uniform result very quickly attained. It becomes a pleasant and interesting occupation to produce a minor. But two effects have presented themselves in this operation which unfortunately bar the way to the very best results. In the first place, the edge parts of such mirrors for more than half an inch all round bend backward, and become of too great focal length, and the rays from these parts cannot be united with the rest forming the image. In the second place, the surface when critically examined by the second test, is found to have a delicate wavy or fleecy appearaoce not seen in machine polishing. Although the variations from the true curve implied by these latter greatly exaggerated imperfections are exceedingly small, and do not prevent a thermometer bulb in the sunshine appearing like a disk surrounded by rings of interference; yet they must divert some undulations from their proper direction, or else they would not be visible. All kinds of strokes have been tried - straight, sweeping, circular, hypocycloidal, etc. - without effecting their removal. M. Foucault, who used a paper polisher, also encountered them. Eventually they were imputed to the unequal pressure of the hand, and, in consequence, a machine to overcome the two above mentioned faults of manual correction was constructed." Here Dr. Draper proceeds to describe his machine.
The apparatus constructed by the writer was also intended to obviate the unequal pressure of the hand, but at the same time retaining the "feel" of the polisher. This consists (see figure) of two uprights spiked to floor and ceiling. They should be planed on all four sides and set exactly plumb. Against them a horizontal bar is bolted, conveniently spaced bolt-holes in the uprights providing for raising and lowering the bar. The middle of this bar supports an iron plate P, 1/2 in. thick. Through the overhanging part of P, and well clear of the bar, a 1 in. round hole is drilled, which is then countersunk nearly all the way through the plate, the cup-shaped depression being made spherical to receive a smooth iron ball 1 3/4 in. diameter, which by a ground fit is thus uniformly supported, while revolving in any direction. The top of this ball may be squared to give wrench hold. Through one diameter of the ball, a hole is drilled into which is firmly screwed the end of a 3/4 in. round iron rod several feet long, the upper end of which has, for this purpose, been turned or drawn down to 1/2 in. and threaded, thus forming a short neck under the ball.