390 
POPULAR SCIENCE REVIEW. 
enormous difficulty presented itself. To make a speculum of 
the required dimensions it was necessary to have a disc of good 
crown glass about thirty-eight inches in diameter and from six to 
nine inches in thickness. Well, purchase such a disc ; or rather, as 
it was not likely that such a thing could he 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 disc 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 disc was to be turned was en- 
trusted to Mr. Gr. 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 manu- 
factured 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 re- 
maining 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. Firstly, 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. Secondly, 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 sup- 
ported 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. Thirdly, 
arrangements must be made so that an observer can always use 
the telescope, and be enabled to look through the eyepiece of the 
telescope whatever position it may be in — no slight task, seeing 
that the said eyepiece 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 Kosse, and Mr. Lassells, and that at 
Melbourne, it is much easier to satisfy the first condition than. 
