144 
DR. T. R. ROBINSON AND MR. T. GRUBB’S DESCRIPTION OE 
difficulties to a minimum ; and it is quite manageable by any one who has a good head 
and a good eye, with a little training. 
[In addition to the ordinary (metallic) small speculum of the Cassegrain there is 
supplied one of silvered glass, of a construction originally proposed aud described by 
Mr. Grubb in 1857 (see Report of British Association for that year). 
It consists essentially of an achromatized combination of crown and flint glass, the 
outer surface, or that which receives the rays coming from the large speculum, being 
of a curvature which will sufficiently disperse any light reflected from same, its fourth 
surface being made highly reflective by a thick deposit of silver. The inner surfaces of 
the compound are made coincident in curvature so as to admit of being cemented, and 
the refractive and reflective powers of the compound are so adjusted that their sums 
(taking into account that the rays are twice refracted) shall be equal to that of an ordi- 
nary or metallic speculum of the required focus. 
In the supporting of this silvered glass speculum the increased difficulty experienced 
by others of supporting glass as compared with speculum metal has presented itself. 
It has not, however, been found that any particular diameter of the lenses being vertical 
caused a difference in this respect ; perhaps this may be attributed to the glass used, 
having been prepared in the form of disks as generally used in the constructing of object- 
glasses, whereas it has probably occurred in other cases that, as the quality of the glass 
suited to the reflectors of M. Foucault is understood to be by no means material, rolled 
glass has been used, and that therefrom arises a difference in different directions to the 
power of resisting flexure.] 
But it is not enough to have a fine speculum unless it is supported in the telescope so 
that it may be exposed to no stress which can change its figure. A local pressure of a 
very few pounds at the back or even the edge of one of these massive 4-feet specula would 
entirely deform its image ; and such pressure is not easily avoided in the varying posi- 
tions of the instrument. In the first reflecting telescopes, the specula were held by three 
front stops, against which they were kept by three or more screws behind. The screws 
were afterwards replaced by three springs, and this defective system remained till the 
Herschels substituted one unobjectionable in principle. They placed the speculum in 
a strong metal box resting on several folds of soft and elastic cloth. This diffuses the 
pressure uniformly over the back of the speculum, while the box can rest on adjusting 
screws ; if there were as good a lateral support, which could be easily applied, this 
system is (except for very large instruments) excellent. M. Foucault supported his 
specula on air-cushions which were inflated as required ; this would also give very uni- 
form support, but would be affected by changes of temperature, and would, it seems to 
me, be inconsistent with that permanence of index corrections which is essential to the 
purpose of this telescope. It has not only to show nebulae and faint stars, but to deter- 
mine their places to the accuracy of a very few seconds ; and therefore at every hour- 
angle and polar distance, the position of the optic axis of the great speculum with 
