196 REPORT— 1857, 
Now the two points in which inferiority may be at present held as against 
the reflector are—the greater liability of the surfaces to tarnish, and the 
less intrinsic brilliancy of the pencil. In respect of the first, I hold the 
objection to be much less in amount, with good specula, than usually sup- 
posed. If we have to infer that Sir J. Herschel frequently repolished his 
mirrors at the Cape, we know that some specimens of optical glass have 
rapidly deteriorated. It was stated by Professor Moll, of Utrecht, at the 
former Meeting of the British Association in Dublin, that there was then in 
Paris an object-glass through which we might in vain attempt to look ; but 
it is manifest that neither a low quality of speculum metal, nor glass carrying 
its own destruction within its substance is fitted for optical instruments, and 
that both should be equally avoided. As a proof of the permanence of good 
specula metal, [ may mention that on a. recent occasion, a surface twelve 
years polished showed an increase of only six per cent. of reflecting power 
on being repolished. 
In respect of the second point of inferiority of the reflector, viz. the greater 
absorption of the incident light, and consequent lesser intrinsic brightness of 
its pencil, as compared with that of the achromatic. I would observe, in limine, 
that this difference decreases as the size of the object-glass increases, so that 
an object-glass of 4 feet diameter, and of a thickness adequate to resist 
flexure, would transmit little, if any, more light to the eye than a reflector of 
equal aperture as it is now possible to construct it. Such considerations do 
not however lessen the importance of obtaining for the reflecting telescope 
every possible accession as well to the permanence as to the reflective power 
of its surfaces, compatible with their general accuracy and perfection of 
figure. To the improvement of the reflecting telescope in these respects, I 
have lately devoted some attention ; how far I have realized what is valuable 
remains to be shown. 
So far as the Cassegrain and Gregorian forms are concerned, these im- 
provements are based upon the employment of one or more silvered (not 
quicksilvered ) surfaces; and my first application of it has been to that form 
of the reflecting instrument which I have long preferred (not perhaps without 
good reason) to all others, viz. the “ Cassegrain.” Convinced, from previous 
practical working of both speculum metal and glass, that both were capable 
of receiving equal degrees of accuracy of surface, I conceived it unnecessary 
to stop to consider whether the failure of a recorded attempt to construct a 
reflecting telescope of quicksilvered surfaces was due to the errors of work- 
manship of the artist, or the formula by which he was guided, and selected 
the small mirror of the Cassegrain telescope for experiment. 
Now the most obvious construction for a silvered mirror for such, was to 
form a lens (so to speak) of equal thickness throughout, having no disper- 
sion, and therefore requiring no correction of colour, and to silver the con- 
cave surface. This construction I rejected, notwithstanding its simplicity, 
on considering that there would be a secondary image (coinciding nearly 
with the primary) formed by the outer or unsilvered surface, aud producing 
what is called a “ ghost” in the field of view. 
I therefore assumed a radius of curvature for the outer surface differing 
considerably from that of the inner or silvered surface; and as this would 
produce refraction and therewith colour, it became necessary to adopt an 
achromatized compound of crown and flint glass. This being constructed, 
has proved altogether satisfactory : the inner surfaces being cemented, no ap- 
preciable loss of light occurs from using two lenses instead of one; the re- 
flecting surface being as yet only quicksilvered, no increase of light should 
be expected: still, when the combination is used in a telescope, the image 
— 
