VIII. On the use of Silvered Glass for the Mirrors of 
Reflecting Telescopes. 
ByvG: B. AIRY, B.A: 
OF TRINITY COLLEGE. 
FELLOW OF THE CAMBRIDGE PHILOSOPHICAL SOCIETY. 
[Read Nov. 25, 1822.] 
Tue idea which probably first occurred to the inventors of 
reflecting telescopes was that of constructing their reflectors of 
silvered glass; but it appears to have been immediately rejected. 
The difficulty of grinding and polishing metallic specula was then 
so great, and the formation of glass reflectors so easy, that some 
serious disadvantages must have appeared to be connected with 
the use of the latter, or Newton would not have bestowed so 
much labour on the construction of metallic mirrors. The princi- 
pal objections appear to have been these: if the surfaces were 
ground to equal radii there would be a confusion of the images 
produced by reflection at the two surfaces of the glass: if ground 
to different radii, refraction would be introduced, and consequently 
dispersion, which it was intended by the construction of the 
reflecting telescope to avoid. Besides these, the loss of light by 
reflection from glass is perhaps greater than that by reflection 
from polished metal. For these reasons, it would seem, the use 
of glass reflectors has been entirely neglected, and opticians have 
endeavoured to improve the reflecting telescope only by improving 
the composition of the speculum-metal. 
Vol. If. Part 1. O 
