LORD OXMANTOWN ON THE REFLECTING TELESCOPE. 
521 
about 150°, and the soft mixture laid on with a large flat brush, to about the thickness 
of one-thirtieth, or one-twenty-fifth of an inch; it is then suffered to cool to about 100°, 
and the hard mixture applied in the same way and to about the same thickness. When 
the temperature has sunk to 80°, the polisher is placed on the speculum previously 
covered with peroxide of iron and water, of about the consistence of thin cream. It 
may be well to observe, that the speculum is not endangered by applying a polisher 
at 80°, while it is but 55°, because the thin layer of resin retards very much the trans- 
mission of heat ; but in grinding, where there is no resin, were there such a disparity, 
the speculum would be broken. In grinding the three-feet speculum formed in 
separate pieces, the iron plate happened to have been washed with warm water, and 
though but little warmer than the speculum, the moment it was put on several of 
the plates cracked, but from the construction it was but little injured : had it been 
the other speculum in one piece, it would of course have been destroyed. 
I prepare the peroxide of iron by precipitation with water of ammonia from a pure 
dilute solution of sulphate of iron ; the precipitate is washed, pressed in a screw press 
till nearly dry, and exposed to a heat which in the dark appears a dull low red. The 
only points of importance are, that the sulphate of iron should be pure, that the water 
of ammonia should be decidedly in excess, and that the heat should not exceed that I 
have described. The colour will be a bright crimson, inclining to yellow. I have 
tried both potass and soda pure instead of water of ammonia, but after washing with 
some degree of care, a trace of the alkali still remained, and the peroxide was of an 
ochrey colour till overheated, and did not polish properly. Thomson says the per- 
oxide of iron is sometimes of an ochrey colour, probably owing to some impurity, 
and I have found that the slightest trace of potass or soda produces that effect. 
Possibly even washing with a degree of care, too troublesome for practice, would be 
ineffectual in removing the last remains of the alkali, as Davy found that silica pre- 
pared with an alkali always retained a trace of it, even after the most careful washing ; 
but this is not exactly a case in point. 
Having thus endeavoured to point out as concisely as possible what I have found 
essential in producing a very true surface and a fine polish, without at all wishing it 
to be inferred that I consider these processes quite perfect, — for, on the contrary, I 
believe much still remains to be accomplished, — I will next describe the means by 
which I have endeavoured to obtain a very good general figure. 
When I had but little experience in working specula, considering the subject more 
theoretically than practically, I thought that a spherical figure was the only one which 
could be wrought with sufficient accuracy for optical purposes, and therefore that an 
original spherical aberration was an unavoidable evil ; but that were it possible by any 
counteracting means to neutralize, or even diminish it, we might have telescopes of 
greater aperture with a given focal length. I constructed, therefore, a Newtonian tele- 
scope of six inches aperture, and two feet focus* : the speculum was in two concentric 
* Sir David Brewster’s Journal, July, 1 S 28 . 
3 x 
MDCCCXL. 
