342 THE POPULAR SCIENCE MONTHLY. 



of the glass speculum, and here at the outset an enormous difficulty pre- 

 sented 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 neces- 

 sary 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 re- 

 flecting telescopes of Lord Rosse and Mr. Lassells, and that at Mel- 

 bourne, 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 



