of the glass speculum, and here at the outset an enormous difficulty presented itself. To make a speculum of the required dimensions it was necessary to have a disk of good crown glass about thirty-eight inches in diameter and from six to nine inches in thickness. Well, purchase such a disk; or rather, as it was not likely that such a thing could be bought ready-made, why order one. This seems feasible enough. But there was not a firm in England who would undertake to make such a thing. In fact, at the time, the opinion was freely expressed that such a thing could not be made. This was a serious obstacle, for nearly all the glass used for optical purposes came from England. Determined not to be baffled, Mr. Common applied to a French firm, and they produced the disk of glass which was essential before a single step could be taken. The first difficulty was faced and overcome.
After mature consideration the grinding and polishing of the speculum into which this glass disk was to be turned was intrusted to Mr. G. Calver, of Widford, a well-known maker of glass specula. From its enormous size, over twice as large and ten times as heavy as any speculum which had ever been manufactured before, it was necessary to construct new and more powerful machinery and even a new building. Nothing daunted, however, Mr. Calver agreed to do his best to turn this great mass of glass into an excellent speculum, though of course he could not guarantee anything, the entire risk necessarily remaining with Mr. Common.
This settled, the greater portion of the task remained to be faced. Given a speculum of the specified size, how was it to be mounted, and how was it to be used? 1. The glass speculum must be mounted with such care that, despite its enormous weight, it must nowhere bend by as much as one ten-thousandth of an inch. 2. The glass speculum and the iron cell which supports it must be fastened at the end of a tube some twenty feet in length, and this tube must be supported by an elaborate mounting by which it can be pointed to any desired part of the heavens, and moved by clockwork so as to follow the apparent motion of the celestial bodies. 3. Arrangements must be made so that an observer can always use the telescope, and be enabled to look through the eye-piece of the telescope whatever position it may be in—no slight task, seeing that the said eye-piece must in some positions of the instrument be over twenty feet from the ground. Lastly, the telescope must have an observatory which will shield it from the weather, and yet permit any part of the heavens to be examined with the telescope.
When the instrument has a metallic speculum, like the large reflecting telescopes of Lord Rosse and Mr. Lassells, and that at Melbourne, it is much easier to satisfy the first condition than when the speculum is made of glass; for it is possible to cast the speculum with grooves, projections, and recesses in its back, by means of which the task of supporting it is much simplified. With a glass speculum it is
