516 
LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
the one I have described is by far the best. The wheel B makes, when polishing a 
three-feet speculum, sixteen revolutions in a minute ; to polish a smaller speculum the 
velocity is increased by changing the pulley on the shaft A. The machine is in a 
room at the bottom of a high tower, and doors can be opened in the successive floors, 
so that a dial-plate of a watch placed perpendicularly over the speculum can be ex- 
amined at any moment. The dial-plate is attached to a mast, so as to be much higher 
than the tower and about ninety feet from the speculum ; and a small flat metal and 
eye-piece, with its proper adjustments, completes the arrangements for a Newtonian 
telescope. This simple contrivance has greatly facilitated the progress of these ex- 
periments. As appears in the plate, all the motions are produced by bands three 
inches wide, instead of the more permanent gearing of cog-wheels : although bands 
are liable to break, I think they are preferable to cog-wheels, because in a machine 
like this, which is for experimental purposes, if any part should become fast, which 
has happened more than once, the band falls off or breaks, and no mischief is done : 
it is only in a manufactory, where there are no experiments, but merely a routine of 
unvaried operations, that such accidents can be guarded against. One-horse power 
is quite sufficient to drive this machine while it is working a speculum of three feet 
diameter. The engine, however, is three-horse power, as from the distance of manu- 
factories, it has been necessary to execute all the turning and casting work in my own 
laboratory. 
The first serious difficulty which presented itself in polishing specula of consider- 
able size, according to the common process, was this: when the layer of pitch was 
thin, as it must be to produce a good figure, however accurately at first it fitted the 
speculum, it soon ceased to do so, and the polishing did not of course proceed pro- 
perly. This derangement, which in the ordinary mode of polishing by the hand is 
perceived at once by the feel, is as soon perceptible with the machine, because mi- 
nute bubbles from the air, which has insinuated itself between the speculum and the 
polisher, are immediately observable. For some time this difficulty was exceedingly 
puzzling, and it was not until after many abortive attempts that the cause became 
evident : during the operation of polishing, the abraded matter, mixed with the polish- 
ing powder, is in part taken up by the pitch, but not equally over the whole surface ; 
as, however, pitch is not sensibly elastic with a moderate pressure, wherever most is 
taken up, there the surface will be most prominent, and the figure of the polisher de- 
stroyed, unless the pitch can spread laterally. To allow of this lateral expansion a 
certain thickness of pitch is necessary, and I found, as might indeed have been an- 
ticipated, that the thickness required to be increased with the size of the speculum ; 
in fact, if 1 may be allowed so to express myself, that the necessary thickness was 
some function of the diameter of the speculum. By using a layer of pitch sufficiently 
thick, a solid speculum of twenty inches aperture and a divided speculum of twenty- 
four inches aperture, the subjects of these and many other experiments, were made 
to define tolerably with a low power, and, at the same time, had acquired a high polish. 
