SPECULA OE REFLECTING TELESCOPES. 
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77 teeth and the lower wheel 60 teeth. These are shown in position in figs. 1 & 2. 
Frame D, shown in the several figures, carries a tangent-screw (figs. 3 & 4) working into the 
upper wheel of the shaft (fig. 5), the shaft of the tangent-screw having three speed-pulleys, 
the largest of which is 9 inches diameter, keyed on to it at its end, distant about 20 inches 
from the middle of the tangent-screw. The lower wheel of the shaft (fig. 5) engages 
with a smaller wheel or pinion of 26 teeth, in the lower end of the shaft of which is cut 
a slightly dove-tailed slot (G, fig. 1). Through this slot passes the adjustable crank-arm 
(E, fig. 2), with a turned pin and shoulder at its end. The stout bracket F is bolted to the 
underside of B, and contains a planed groove, vertically cut, which is fitted by a brass 
step bored to the size of the crank-pin, and travelling truly and smoothly in the slot or 
groove cut in the bracket. The extreme throw of this crank is, radially, 2’2 inches; 
therefore the entire journey of the plate C, with all that it carries, along the V-grooves 
in B is thus insured to the extent of 4 ’4 inches extreme thrust to and fro. Cast on to 
the upperside of the tangent-wheel H are a central and three circumferential bosses, 
seen in plan in fig. 4, and in elevation in figs. 1 & 2. The projecting pin of the central 
boss enters a hole of similar size in the centre of the back-plate of the speculum, on 
which, on its disks and levers, it reposes. This central boss thus secures the centrality 
of the speculum whenever it is placed upon the machine. The steadiness and horizon- 
tality of the back-plate is secured by three adjusting-screws affixed to the other three 
bosses (of which one is seen in fig. 1), having pins entering corresponding holes in the 
back-plate. By a band from a suitable-sized pulley on the main driving-shaft of the 
steam-engine, motion is given to the pulley on the tangent- screw shaft (I, fig. 1). 
This being engaged with the wheel of 77 teeth, causes the speculum to revolve on its 
axis, and at the same time, by means of the wheel of 60 teeth working into the pinion 
of 26 teeth, carries the speculum transversely or laterally along the Y-shaped grooves, 
according to the setting of the adjustable crank-arm E, fig. 2. The object of this trans- 
verse motion (not always used or even necessary) is to wipe out (so to speak) any ring- 
like character which might possibly appear in the process of polishing. 
Thus far is a description of the apparatus by which the two motions of the speculum 
are obtained. I proceed now to describe the method by which the required motions of 
the polisher or grinder are secured. 
Description of the Apparatus for driving the polisher or grinder of a two foot Speculum. 
Fig. 7 of Plate 51 is a plan, and fig. 8 an elevation, of this part of the machine. A 
represents the speculum as placed in position on the bosses of the tangent-wheel H, 
figs. 1 & 2. B is the principal spindle, with its adjustable crank for driving the long 
shaft C, which, seen in its two positions (figs. 7 & 8), needs but little description. D is 
the main driving-pulley, which, in connexion with a shaft running along the ceiling, 
also driving the speed-pulley I (fig. 1), gives motion to the whole. E is another vertical 
spindle attached to the wall of the laboratory, furnished also with a crank of nearly 
similar range to that on the spindle B, and connected also, by a radial bar, with the 
