1868. | Description of the Great Southern Telescope. 455 
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the speculum box (which serve for levelling the mirror), the points 
of which carry levers (primary system) supporting triangles on their 
extremities (secondary system), from the vertices of which are hung 
two triangles and one lever (tertiary system). All the joints of 
this apparatus are capable of a small rocking motion, to enable 
them to take their position when the speculum is laid upon them. 
In the system of those levers made by Lord Rosse for his six-feet 
speculum, the primary, secondary, and tertiary, &c., systems were 
piled up one over the other, so that the distance from the support 
of the primary to the back of the speculum was about 15 inches. 
This, as will be readily seen on consideration, introduced a new 
strain when the telescope was turned off the zenith, and had to be 
counterpoised by another very complicated system of levers. But 
in this telescope, by the substitution of cast-steel for cast-iron, and 
by hanging the tertiary system from the secondary and allowing it 
(the tertiary) to act in some places through the secondary, the 
whole system is reduced to 34 inches in height, and the distance 
from the support of the primary lever to the back of the speculum is 
only 13 inch, by which means this cumbersome apparatus is entirely 
done away with. 
The ultimate points of the tertiary system are gun-metal cups 
which hold truly ground cast-iron balls with a little play, and when 
the speculum is laid on these it can be moved about a small quantity 
. by a person’s finger with such ease as to seem to be floating in some 
liquid. 
The System of Lateral Supports was devised by Mr. Grubb 
specially for this instrument. One objection originally raised to the 
mounting of these large telescopes (reflectors) equatorially, was 
that such heavy mirrors could only be supported daterally in a 
