We have now got rid of the surface produced by the coarse emery, and the figure of the speculum is beginning to approximate to a sphere. The object of the fine-grinding is to perfect the spherical figure as far as possible, and to render the surface so fine that it can be readily polished. The fine-grinding is commenced with No. 120, 150 or 180 - it does not much matter which - and continued in the same manner as before until all trace of the " 49" surface is gone. It will be found a great convenience to apply the emery to the tool by means of a flat, soft brush. The brush is dipped in water, then into the emery, and the small quantity it takes up is painted evenly over the tool. There is little fear of sticking at this stage; but if it occurs it is a sign that the emery is not spread over the tool sufficiently even. The stroke is kept irregular, as already indicated. It should not take long to get rid of the 46 surface, and, as I said in my last, it is important to do so completely. Any impatience at this stage of the work will render the final surface difficult to polish. After the 130, washed flour emery is used in the same way and with a similar stroke.

The use of the brush to apply the emery is even more to be recommended with the washed flour. If care is taken to have the emery quite evenly distributed, so that actual contact of the glass surfaces does not take place, sticking can be completely avoided. A little saliva, or a small quantity of soap in the water, will help to this end.

The " washed flour" produces a very fine surface, but not nearly sufficiently fine to polish; so it is necessary for the worker to prepare his finer emeries himself. My way was as follows:

Take 2 lbs. or 60 of washed flour emery and stir it up in a jug with half a gallon of water, then leave it to stand for one minute. The coarse emery will sink to the bottom, leaving the finer sorts in suspension ; after the minute has elapsed, the water, holding in suspension the emery required, is poured or siphoned off into a basis, where it is allowed to settle for several hours. The water is then poured off and the emery may be dried and collected. The process is repeated, except that five minutes are allowed to elapse before pouring; then 15 or 20 minutes, then 40 minutes. It is possible to go even further, but I found that little was gained. We thus abtain a series of washed emeries of increasing fineness, the quantity obtained, of course, decreasing as the length of time during which the emery is allowed to remain in suspension increases. Very little of the finer grades is, however, required, and 4 lbs of washed flour should prove ample for a 9 in. Of course the "one minute " and "five minute " not required can be returned to the washing jug. Care must, of course, be taken that none of the sediment in the washing jug comes over with the liquid. A siphon of india-rubber tubing is distinctly useful.

In practice It is not necessary to dry these emeries, the superfluous water having been drained off; they can be applied to the tool with a soft, flat, camel's-hair brush, and will contain just about the right amount of water required.

It will be seen that I used 1, 5, 20 and 40-minute emeries. There is no virtue in these figures, all that is required being to secure a steady diminution in the size of the grains.

After each grade is finished with, it is a good plan to add water to the emery left on the tool, and grind for a few minutes with the small quantity of emery left. This tends to fineness of surface.

Here a word of warning may not be out of place. The motion of mirror over tool with the fine emeries is exquisitely smooth and frictionless, and the mirror should never be left to itself on the tool without a hand to hold it, otherwise it is very liable to slip off with disastrous consequences.

Some workers recommend that the tool should be divided into squares, by grooves either ground or filed out. This is said to prevent sticking, and in large work is, I believe, essential. But I did not find it necessary for my 9 in., and in any case it would be a tedious job to cut the grooves. The fine-grinding is is not the least fascinating part of the work, the surface produced being so exquisitely fine it has been compared to the side of a tumbler in which milk has just been, and should be semi-transparent, so that a candle flame can be seen at a distance of 10ft. or more. In my case, after the fine-grinding was complete, large type could be read through the speculum placed on its back, although the glass is 1 1/2 in. thick.

The importance of obtaining a really fine surface before polishing cannot be over-estimated. It is impossible to get a really good polish unless the surface is properly prepared, and each grade should be used until it fails to produce any further effect. Moreover, if care is taken over the fine-grinding, the surface produced will be almost perfectly spherical.

On the completion of the fine-grinding the wooden disc and handles should be removed. This is easily done by standing the mirror on its edge, holding it firmly, and striking the edge of the wooden disc with a mallet. Any pitch left on the back of the mirror can be scraped off and finally washed off with turpentine. - "English Mechanic. "