M. A. AINSLEY

The telescope I propose to describe was made some two years back, and since it was finished several improvements and alterations have occurred to me - these I will indicate as I proceed. I will devote this letter to a brief summary of the methods I used and the materials required.

The speculum (and this applies to the second 9 in. mirror also) was worked by hand on a glass tool of the same size, the latter being supported on a stout box which was fixed to the top of a cask. The speculum was cemented with pitch to a wooden handle until the fine-grinding was completed, when it was held in a tight wooden triangle, just tight enough to hold it without undue pressure or freedom. The mirror was figured on a graduated polisher (to be described later) and tested by the Wassell method, though my apparatus was considerably simpler than Mr. Wassell's. Having nothing in the way of a workshop (in fact, a good deal was done out of doors), and only a few hand tools, I was largely dependent on the local carpenter and blacksmith, so that anyone with a small workshop and some skill as a carpenter would make a material reduction in the cost.

As to the materials, the glass will be the first thing to get. I was advised to have my 9 in. mirror 1 1/2 in. thick; but I found a good deal of difficulty in procuring glass of this thickness, and had to be content with 1 in. This has proved ample in practice - with care in supporting the mirror, to avoid flexure - but as Dr. Common said in a letter to me: "If your 9 in. were 1 1/2 thick it would not suffer from flexure, however supported. " So I should advise 1 1/2 in., or about one-sixth of the diameter of the mirror. I expect 1 1/2 in. would be thick enough for any size up to l0 1/2 in. or even 12 in. They should be roughly ground circular, and the sharp edges removed. Fine grindimg of the edges is unnecessary.