Throughout the grinding operations I used ordinary emery. 1 understand that "carborandum " is a much more effective grinding material, and I am told it cuts far more rapidly; but it is much more expensive, and as yet I have not tried it; but if I do any more work of this description I shall certainly do 60.

For the rough grinding, emery of the grade known as "1 1/2" was used. Sand is a good material for the purpose. In fact, it is recommended by Dr. Common ; but I had no supply on hand, and I have found from past experience that the use of it lengthens the rough grinding considerably. It cuts well when quite fresh, but very soon seems to wear down and so has to be very frequently renewed. I tried rough-grinding with coarser emeries, even as coarse as No. 4 1/2, but the edges of both tool and mirror were badly chipped; and it appears also that if the rough-grinding material is too coarse, the mirror, after "roughing out" is not spherical, but has considerably more curvature in the center. If a finer grade of emery than "1 1/2" is used, the rough-grinding is much prolonged.

About 10 pounds of the "1 1/2" emery will be required for the "roughing-out" of the mirror. The amount required, of course, depends on the diameter and focus of the mirror, and it is not to be expected that the process should be as rapid as in the case of a mirror of smaller size. The depth of the center of my first 9 in. mirror is about 1-12 in., the focus being 63 in., and it is obvious that as the depth of the mirror in the center varies as the square of the aperture, the focus being constant, the time being taken to dig out a mirror of large diameter is much greater than that required for one of smaller size. For the fine-grinding, emery of No. 46 grade was used first; when the curve became nearly spherical, emery of No. 150 grade was employed, then washed flour emery; after which I prepared the finer emeries by processes to be described later. The amount required for the 9 in. mirror was as follows: No. 1 1/2, about 10 lb.; No. 46 about 2 lb.; Nos. 120 or 180, or anything of about that fineness, about 2 lb.; washed flour emery about 6 lb., though more may be required if the later stages of fine-grinding have to be repeated.

The exact grade of emery required for each stage is not very important. For example, an equally good result could be obtained with emery of say, 1 1/2, 80, 180 as with 1 1/2, 46,180, the main point being to insure a steady diminution in the size of the grain. There is no necessity to use many different grades; in fact, Dr. Common used sand 150, and worked flour in succession. I quote from a letter received from him about two years ago.)

I will defer any consideration of polishing material till later. Meanwhile I would impress upon the would-be beginners in the art the necessity for patience in each stage of the proceedings. I can speak from experience that more trouble is caused by hurry and impatience in going from one grade to the next than by anything else. " The greatest thing in mirror-grind" ing is to get a good surface before polishing." (Dr Common.) And I have found much reason to believe in the truth of the statement. Every stage of the work must be thoroughly done, and ' More haste less speed" is never truer than in this delicate and fascinating art.