Ieee one tel * i 
TRANSACTIONS OF THE SECTIONS. 4 
tion of celestial objects. It is certain that at this moment, and despite the multi- 
plied improvements in the manufacture of large glasses, the most powerful instru- 
ment directed towards the heavens is a telescope with a metalspeculum. The tele- 
scope of Lord Rosse is 6 feet English in diameter, and its focal distance is 55 feet. 
Possibly the reflecting instruments would have gained the superiority, could the 
metal take as durable a polish—could it be as well worked as the glass, and were 
it not heavier. Placing thus in parallelism the two sorts of instruments, and dis- 
cussing their respective qualities and defects, I finished by conceiving that the tele- 
scope with a glass would possess every advantage, if the mirror being once shaped 
and polished we could communicate to it the metallic brilliancy, in order to obtain 
from it images as ]uminous as those of the refracting telescopes. This thought, 
which at first appeared a fiction of imagination, was soon converted into a satisfac- 
tory reality. The glass being cut by an experienced optician, and thoroughly 
polished, is ready to be covered by Drayton’s process with a very thin uniform coat 
ing of silver. This metallic coating, which when taken out of the bath in which it 
is formed is dull and dark, is easily brightened by rubbing with a skin lightly tinged 
wth oxide of iron, and acquires in a short time a very brilliant lustre. By this 
operation the surface of the glass is wholly of metal, and becomes vividly reflective, 
not exhibiting under severest tests the slightest alteration in form. To procure a 
disc of glass with concave surface perfectly finished, I applied to Mr. Secretan, who 
had the kindness to provide for me a clever workman. On the other hand, to be 
able to obtain a deposit of silver, I had recourse to the owners of the English patent, 
M. Power and M. Robert, who actually work the process in France, and who fur- 
nished me with the silvery solution, giving at the same time the fullest instructions 
how I might best succted. My mirror being silvered, and having acquired a 
polish of steel, I formed a telescope of it of ten centimetres diameter and fifty cen- 
timetres focal length. This little instrument supports well the eye-glass, which 
‘magnifies 200 times, and compared with the reflecting telescope of one metre, gives 
avery sensibly superior effect. Wishing to learn the proportion of light usefully 
reflected by the layer of silver deposited on the glass, and afterwards polished, or, 
at least, to compare the intensity of a pencil of rays reflected by a surface thus pre- 
pared with that of one transmitted by an equal surface from the object-glass of a 
refracting telescope, I accomplished the matter without difficulty by means of a 
photometer with divisions, which I had employed on another occasion. The result 
of this operation ensures a decided advantage to the new telescope. The pencil of 
‘rays reflected on the silvered glass is equal to 90 per cent. of those transmitted 
through an object-glass of four partial reflexions; so that the new instrument avails 
itself of the overplus of light, which, on account of the greater diameter of the mir- 
ror, concurs efficiently to the formation of the focal image. Diameters equal, the 
telescope with glass is by one-half shorter than the other instrument; with equal 
lengths, it bears a double diameter, and collects three and a half times more light. 
Considered in another point of view, the new combination is distinguished in this, 
that it produces all its effect without the concurrence of those numerous conditions 
required to obtain a certain degree of perfection in any telescope, whether reflect- 
ing or refracting. The achromatic telescope, above all, requires that the con- 
structor of it, at one and the same time, pay particular attention to the homoge- 
neity of the two sorts of glass which form the object-glass, their refracting and di- 
spersive powers, the combination of -curves, the centring and the execution of four 
spherical surfaces.. In the new telescope, on the contrary, the glass, serving not as 
a middle refractor, but only to support a very thin layer of metal, the homogeneity 
of the mass is by no means required, and the most ordinary glass of sufficient thick- 
ness worked with care affords a concave surface, which when silvered and polished 
furnishes of itself and by reflexion excellent images. There is one strong objection 
to the metal mirrors,—it is, that they become oxidized in time, and are tarnished 
by contact with the air. Eight months I have kept silvered mirrors, which have not 
yet undergone any sensible alteration. Will they preserve this state of perfection a 
still longer time? The experiment has not been sufficiently prolonged to decide one 
way or the other; but even should the lustre of the mirror become weaker, there 
is no difficulty in recurring to the same means for re-establishing it, by which it 
| _ had been at first obtained. In fine, should the depth of the silver be altered, the 
