THE GrEEAT MELBOURNE TELESCOPE. 
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thing but a spherical figure, which is quite unfit for a speculum ; for one might expect 
that the acting surfaces would wear into uniform contact, and therefore uniform curva- 
ture. But both the speculum and its polisher are elastic, and allow the contact to con- 
tinue, notwithstanding a minute difference of curvature ; how minute appears from this, 
that even at the edge of one of these 4-feet specula the distance of its parabola from the 
circle is only 0-000106. 
This being possible, we can increase the abrasion at various parts of the surface till 
the desired figure is attained. For a long time it was believed that this could only be 
successfully done by the hand feeling the action, but Lord Rosse found that it could 
be performed as well, and with more certainty, by machinery. In this he has been fol- 
lowed by others, and though their methods differ, the general principle is the same. 
The speculum revolves slowly on its axis, while the polisher traverses it more rapidly, 
describing a track which is some continuous curve, and crosses it in every possible direc- 
tion, not returning to the same place till after a great number of revolutions. In Lord 
Rosse’ s machine the motion is the resultant of two nearly rectilinear and at right angles, 
one less and of much slower period than the other. Latterly he reverted to an earlier 
plan of his own, and used a motion which is nearly elliptic. He also surrounded the 
speculum with water of a given temperature. Mr. Lassell and Mr. W. De La Rue use 
an epicycloidal motion, given by a mechanism like that of Suakdi’s pen, which is very 
effective. Mr. Gkubb many years ago made one which combines the power of both 
these*, but gave it up for the simpler one described here, which is remarkable for the 
precision and smoothness of its action. 
[Plate IY. fig. 9 is an isometrical view of the grinding- and polishing-machine. A, A 
are strong A-shaped castings connected by three collared stay-bolts and nuts, and having 
Y-shaped bearings at top, in which turn the circular ends, or trunnions, of the hollow 
prism-shaped casting B. This casting is bored through its centre, and a circle on its 
upper side truly faced ; to it is fitted a spindle with a large face-plate, whose under 
surface applies to the trued surface of the hollow beam for steadiness, and which serves 
to carry the speculum. The lower end of the spindle carries the wheel C, by which 
circular motion is given to the speculum, C being driven by the wheel and pulley D, by 
means of a belt driven from the shaft. 
To the principal framing are attached the brackets E, E, which carry the plate F F ; 
this plate can be raised or lowered to suit different heights, and it carries the horizontal 
shaft G, which, by means of two pairs of bevil wheels of equal numbers, drives the two 
vertical shafts with their cranks H, H. 
These cranks are adjustable to any desired length of stroke from zero, and give motion 
to the connecting-rods I, I, which rods are adjustable as to length. 
The rods I, I act conjointly upon the lower end of the vertical bar K, and this bar 
entering a central hole in the grinder (or polisher), produces the requisite horizontal 
movements in either process. It may be seen that by the adjustment of the strokes of 
* See for descriptions of these the Art. “ Speculum” in Nichoi’s Cyclopaedia. 
