THE GREAT MELBOURNE TELESCOPE. 
145 
respect to the tube must be almost invariable. This should also be the case with the 
small mirror ; but there the difficulty is trifling. For the other, nothing has been devised 
more efficient than the arrangement of equilibrated lever and hoop lateral support. 
[The first is illustrated by Plate V. figs. 10, 11, & 12. The speculum-box consists 
mainly of three parts, A, B, and C, fig. 10. A being a strong-ribbed casting, the left- 
hand side of the figure showing a section through one of the three main ribs ; B is a 
wrought-iron hoop fitting to A, the contact parts being trued in the lathe, and connected 
by a number of screws ; and C is a trued casting fitting the upper part of B, and attached 
to it also by screws. This construction has the advantages (in addition to that of great 
stiffness), 1st, of admitting of the grinding and polishing processes being performed 
without disturbing the speculum, or raising it in its box, the removal of the upper part C 
being sufficient ; 2nd, by the removal of the hoop B, of permitting free access to be 
obtained to the system of levers &c., forming the vertical support of the speculum. 
This system of vertical support is in principle the same as that applied by Mr. Grubb 
in 1834 to a 15-inch speculum constructed for the Armagh Observatory, and subsequently 
used by Lord Bosse, Mr. Lassell, &c., for larger specula ; but in the present instance it 
has probably been carried out more perfectly in its details, and these therefore appear to 
deserve a particular description. In applying this system to 6-feet specula by Lord 
Eosse, and to 4-feet specula by Mr. Lassell, it was considered necessary, partly from 
the considerable weight of the levers, and partly from the distance between the plane 
of the upper ends of the three mam adjusting screws, and that of the back of the spe- 
culum (this distance being in the case of the 6-feet specula 14 inches), to apply a secon- 
dary system of supports to the levers themselves* ; and whatever amount of inconvenience 
arises from this cause, where one diameter of the speculum is always retained nearly 
vertical, a much greater amount would arise where that condition is no longer to be 
retained. 
The whole lever-apparatus has therefore in the present instance been constructed of 
steel, by which its necessary weight is reduced to a minimum ; and secondly, by an inge- 
nious construction which at first presented much difficulty, and which was ultimately 
worked out by Mr. Howard Grubb; the secondary levers of the system, instead of 
being as usual placed between the primary and tertiary, are placed above the latter, or 
nearly touching the under side of the speculum, the balls of the tertiary levers acting 
(where necessary) through holes in the secondary. The result of this is that the distance 
between the back of the speculum and the bottom of the box is only 3-| inches, and any 
necessity for counterpoising the weight of the supporting levers is removed. 
In carrying out in detail this system of support no sacrifice of principle has been 
admitted. The speculum has been supposed to be divided vertically into forty-eight 
portions of equal weights contained in three annuli, viz. two outer of eighteen pieces 
each and one inner of twelve, each single portion being supported under its own centre 
of gravity. Plate V. fig. 11 is a diagram of this supposed division, and fig. 12 shows 
* Mr. Lasseel’s elegant arrangement is described in British. Association Reports, 1850, p. 180. 
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