M. A. AINSLEY

Medium Grinding

We will suppose now that the mirror is roughened out, and that the depth of the concavity in the center is judged sufficient. It is hard to say how long this should take. My own mirror took well over ten hours to rough out, but with carborundum and a smaller mirror the work would be far quicker. However, my first mirror was of rather short focus - only7 diameters - and I certainly should not recommend less than 8 1/2 or 9 diameters for focal length.

The first thing is to get rid of every trace of the rough emery. The tool, mirror, handle, bench, etc., must be thoroughly rinsed and filled up with fresh water. The utmost attention to this is absolutely necessary every time the grade of emery is changed; otherwise the final surface is sure to show scratches which, if not appreciably affecting the performance of the mirror, are unsightly and unworkmanlike.

Having everything clean and free from grit, we "carry on "with No. 46, or No. 80, or something of the sort. It is not necessary to use so much at a "wet, " and at this point we may shorten the stroke and introduce side motion. In fact, I have obtained the best results as regards freedom from sticking and regularity of curve by keeping the stroke quite irregular, sometimes circular, then straight for a bit, then elliptical, then spinning the mirror on its center for a second or two, and then perhaps returning to a short, quick, straight stroke with a little (1 in. or so) side motion. This is a matter on which there may be several opinions; but I can only say that in working my second 9 in. mirror I never had the semblance of a " stick " from beginning to end of the fine-grinding, and the spherical surface- produced left nothing to be desired, both as tested optically and by the spherom-eter. The grinding is continued with No. 40 until the surface made by the coarse "roughing-out" emery is replaced by a uniform surface due to the 46, all coarse pittings having disappeared. This will not take long; but it is absolutely necessary to get rid of every trace of the rough surface. I have found it a good plan to have a series of plate-glass discs ground with the different emeries used, one ground with " 1 1/2, " one with "46," one with "180," and one with " washed flour." The surface of the mirror can be compared with these as the work proceeds, and with the help of a pocket lens it is easy to see if the surface is a true " 46 " or "180 " surface, and so on, or if any trace of the previous surface remains.

When the " 46 " surface is perfect all over, we may wet the mirror and test its focal length, either, preferably, by direct measurement in the sunshine, or by observing the reflection of a candle flame. If the mirror is made thoroughly wet and kept so, as Mr. Ellison recommends, and the image of the sun received on a visiting card, the position of the card where the image is smallest will give an approximate focus, and the focal length can be measured by means of a tape or a piece of string. If not short enough we must continue the rough-grinding a bit further; if too short, the position of tool and mirror should be reversed for a bit; but if care is taken, this should not be necessary, and it may be repeated that the focal length does not matter so long as it is less than seven or eight times the diameter of the mirror. 1 will leave the mirror at this stage for the present. My next will deal with the fine grinding.