524 
LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
near that atmospheric changes do not very materially affect the result, is a far better 
object. 
By keeping the same principles in view, a very perfect plane speculum can be 
worked with facility, difficult as that operation has been found in practice. The 
polisher I make use of for a metal about three inches by two, is three inches diameter, 
divided exactly like the large polisher, but with proportionate minuteness ; when the 
metal is polished, it is tested in the usual way by viewing an object alternately by 
direct and reflected vision, with a very good thirty-inch achromatic, the aperture of 
which has been previously contracted to an inch and three quarters. If the metal is 
concave, it is worked with shorter strokes for about half an hour and then tried ; it 
will be found to have become less concave, possibly convex ; in the latter case it is 
to be worked with longer strokes ; thus with the utmost facility a metal can be 
worked alternately concave and convex, and, with a little practice, the limit between 
the two can be hit with such exactness, that even with the severe test of a thirty-inch 
achromatic, no deviation from the plane can be perceived, and the loss of light will 
be the only evidence that the rays have suffered reflexion before their incidence on 
the object-glass. Smaller flat metals I find it better to polish on the same polisher, 
several together, according to their size. 
The telescope is mounted very similarly to Sir John Herschel’s, but in conse- 
quence of its greater size and weight, I have counterpoised both the tube and the 
whole machine, which makes it easily manageable ; so that upon the whole, though, 
with the experience I now have, I believe the mounting might be improved, it is 
sufficiently convenient. I use it as a Newtonian, as I find that, with its large aper- 
ture and short focus, the saving of light by the Herschellian construction is not at all 
an equivalent for the sacrifice of defining power, at least that is the result of my 
present experience ; the indistinctness, however, from the obliquity of the speculum, 
does not appear to me to be so great as I should have expected, considering the size 
of the circle of least confusion ; for this I cannot account. 
To prevent flexure in specula of moderate dimensions, I find it is quite sufficient 
to support them in their box on three strong iron plates, each plate being one-third 
part of a circular area, the same size as the speculum, and a sector of it ; the plates 
rest at their centres of gravity on points fixed at the bottom of the box of the specu- 
lum, and therefore no flexure of the box can affect the speculum. Although the same 
simple means would probably be effectual for specula of the largest size, in supporting 
specula of three feet diameter I have availed myself of the suggestion of a clever 
Dublin artist, Mr. Grubb, and, at the expense of a little more complication, have 
substituted nine plates for the three, resting on points supported by levers, which rest 
on three original points ; and if flexure is thus more effectually prevented, which I 
think it ought to be, the additional workmanship is of no importance. This lever 
apparatus, however, must be exceedingly substantial, quite disproportionately so, 
otherwise tremors would be introduced by it, attended with the worst consequences. 
