In order to hold the mirror comfortably and to avoid touching it with the fingers during fine-grinding, I obtained a disc of hard wood 6 in. in diameter and $ in. thick with a handle 1 1/2 in. thick and 3 1/2 in. long fixed in the center. The glass disc was smeared with turpentine, and a pool of melted pitch nearly as large as the wooden disc was poured on, the glass having been previously roughly levelled. I strongly advise the would-be speculum worker never to omit the precaution of first smearing the glass with turpentine. I omitted it once, with the result that when the rough-grinding was nearly complete and I was lifting the mirror off the tool, I had the mortification of seeing it drop on a brick floor from a height of some four feet and, of course, on the "buttered side ". Result, a chip 2 in. square out of the face and a new disc required. If, however, the glass is smeared with turpentine, there is little fear of such a catastrophe.

Here a word concerning pitch - which will be a good friend or a bad enemy, according as we use it well or ill. The pitch was Sweedish pitch, in 2 lb. cylindrical boxes. To get the pitch out, do not attempt to chip it, but cut clean through box and all with a chisel. The wood, which is very thin, will come away from the pitch easily and leave a block. I cut mine into cakes 1 in. thick and melted them in an iron ladle over the kitchen range. There should be no solder in the vessel used, and a long handle and spout are convenient; but, before all things, do not melt the pitch over an open fire, or in any place where it or its vapor can possibly come in contact with the flame. This warning has been often given before, but it is most important if danger of fire is to be avoided. Having got a pool of pitch about 5 or 6 in. in diameter, place the wooden dise on it without pressure and move it about until the wooden and glass discs are concentric; then press down firmly and leave to cool. It is important that the handle should be accurately centered; this can be tested by rolling the disc of glass along a table and seeing whether the handle rises and falls or is apparently stationary. This adjustment is of some importance, as it ensures correct centering of the concavity.

Provide a flat, broad-bladed knife (a putty-knife, for instance), a old sponge, and a basin or bucket of water, (warm for choice).

Wet both tool and mirror thoroughly and sprinkle the tool evenly and closely with the coarse emery. A few drops more water from the sponge will about give the amount of moisture required. It is impossible to say on paper how much emery is required for each "wet " or how much water. This is a matter for experience. If too little water is used the grinding is stiff and unsatisfactory. Too much water or too little emery will cause most of the grinding material to be driven off the tool. Too much emery will give rise to a thick mud, which will slow down the grinding.

Having placed the mirror (or what will be the mirror) on the tool, grasp the handle with both hands, the fingers lying across the disc of wood and reaching to its further edge. Work the mirror to and fro with long, straight strokes, no side motion being given ; keep the mirror revolving to the right, or left, as may be convenient, by working the fingers along the edge of the wooden disc, and at the same time walk around the cask, in either direction, so as to get around about once in 20 or 25 strokes. The exact speed is immate-rial, but the motion should be continuous. All this will become quite mechanical after a few minutes' work. It will probably be found that a great deal of emery will be driven off the tool; but it can all be collected with the knife and used again, so there is not much waste. The noise made, if the emery is doing its work properly, should be considerable, and it will gradually reduce as the emery wears down. When the mirror glides over the tool nearly silently it is time to put on a fresh " wet ". The mirror is slid off the tool - a band being kept under it to avoid accident - and sponged over, a fresh lot of emery is evenly sprinkled over the tool, a few drops of water added from the sponge, and the work proceeds as before. As a rough guide to the quantity of emery required for each wet, I may say that for a 6 in. mirror I used about as much as would go on a crown piece, heaped up; but this is only a rough idea of the quantity. After every five or six wets the speculum and tool should be thoroughly washed with the sponge.

Before this has been going on long a straight-edge placed across the mirror will begin to show " daylight" in the center, and we must begin to think of some way of testing the depth of the curve. A plan often recommended is to make templets in glass or zinc by mea»s of a bar cut to the radius of curvatures required and used as a long beam compass, the glass or zinc being ground smooth after cutting. A plan I got from a back number of "Ours" and which proved very accurate and simple, is as follows: Find out by measurement how thick a pile of, say, 100 sheets of writing paper is

(I used glazed paper which proved to be exactly 200 to to the inch). Dividing by 100, or whatever the number is, we get the thickness of one sheet; in my case, .005 in. Prepare a series of strips of this about 1/2 in. wide; then if a straight edge of steel is placed across the center of the mirror and strips of paper placed under its center until it just turns about the center rather than about one end, the depth can be easily arrived at by counting the number of strips required. The depth required, the diameter of the mirror being D and the focus aimed at F, is given by the formula D2/16F

Thus, for a 9 in. mirror of 63 in. focus we have the depth in the center = 81/16 x 63 = .08 nearly, for a 6 in.

mirror of 60 in focus 36 / 16 x 60 =.0375, and so on.

It is worth noticing that if the focal length of the mirror is made eight times its aperture, the depth required in the center becomes D2/16.8D or D/128; and if the focal length of the mirror is made ten times the aperature, it isD/160 . Thus a 9 in. mirror of 6 ft. focus would have a depth of 9-128 or 1-14 nearly. I am assuming that the reader has not got the chance of copying an already-worked mirror. If he has the following will be found very simple. Provide a piece of wood (hard for choice) 6 x 1 1/2 x 3/8 in- Drive two 1 1/2 in. wire nails through one end, side by side, and one through the other end, or use screws, it is quite immaterial, so as to form a long isoscles triangle. Through the center pass a screw about 2 in. long. This will form a rough sperometer, which, if not much use for measuring the depth of a curve, is capable of testing with very considerable accuracy when a certain curve is obtained. Place it so that the points of the end screws rest on the center screw until the whole just turns about its center rather than one end. When the rough sperometer is transferred to the mirror being made, the curve thereof is known to be correct when it just turns about its center as before.

When the rough-grinding is carried so far that the depth of the mirror in the center is as required, it will probably be found on testing with the spherometer that the center is somewhat more curved than the parts nearer the edge; this will disappear in the fine-grinding, but if a definite focal length is required, it is advisable to go on rough-grinding a little more, as the effect of the fine-grinding, in my hands, at least, is to slightly lengthen the focus.

The stroke in rough grinding may be as long as can be given without the center of the mirror going beyond the edge of the tool. If the mirror lifts, in consequence of a tool long stroke, chipping is to be feared. A certain amount of pressure with the hands is permissable but if too much is applied, the emery is liable to be driven off the tool.