NATURAL PHILOSOPHY. 223 



ing- 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 

 discussing their respective qualities and defects, I finished by conceiving that 

 the telescope with a glass would possess every advantage, if the mirror be- 

 ing once shaped and polished we could communicate to it the metallic bril- 

 liancy, in order to obtain from it images as luminous as those of the refract- 

 ing telescopes. This thought, which at first appeared a fiction of imagina- 

 tion, was soon converted into a satisfactory 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 coating 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 skirt lightly tinged 

 with 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. Secrctau, 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. Kobert, who 

 furnished me with the silvery solution, giving at the same time the fullest 

 instructions as to how I might soonest succeed. My mirror being silvered, 

 and having acquired a polish of steel, I formed a telescope of it of ten cen- 

 timetres diameter and fifty centimetres focal length. This little instrument 

 supports well the eye-glass, which magnifies 200 times, and compared with 

 the reflecting telescope of one metre, gives a very 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 com- 

 pare the intensity of a pencil of rays reflected by a surface thus prepared 

 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 insures a decided advantage to the new teles- 

 cope. 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 reflections; 

 FO that the new instrument avails itself of the overplus of light, which, on 

 account of the greater diameter of the mirror, concurs efficiently to the for- 

 mation 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 con- 

 ditions required to obtain a certain degree of perfection in any instrument, 

 whether reflecting or refracting telescope. The reflecting telescope, above 

 all, requires that the constructor of it, at one and the same time, pay partic- 

 ular attention to the homogeneity of the IAVO sorts of glass which form the 

 object-glass, their refracting and dispersive powers, the combination of 



