142 ANNUAL OF SCIENTIFIC DISCOVERY. 



was by no means so easy a task as may be supposed ; for, although 

 our eminent silversmiths do produce surfaces of silver of an extremely 

 brilliant polish, as in the magnificent plateaux and other articles which 

 they turn out, yet, if any one will take the trouble to examine these 

 surfaces, they will be found to be so irregular, though highly polished, 

 as to be entirely unfit for producing correct images by reflection. And 

 it is a singular fact that, although in the first part of the process of 

 polishing chamois leather of the finest kind was used to rub the rouge 

 on the silver surface, yet the finer finishing polish had always to be 

 communicated by the human hand. Nor would the hand of every 

 individual answer ; the manufacturer had to select those with the very 

 softest and finest grain ; nor would the hand of perhaps one in every 

 twenty of the persons employed answer for thus giving the final finish. 

 But it was obvious that the irregular action of the human hand would 

 by no means answer the end he had in view. Suffice it to say, that, 

 at length, after many fruitless trials, he had succeeded in producing a 

 polishing surface, which seemed fully to answer the purpose, by expos- 

 ing spirits of turpentine to the continued action of air, or by dissolving 

 a proper quantity of resin in the spirits of turpentine, and, by means 

 of this varnish, applying the rouge to the same description of polishing 

 substance which he used in polishing the speculum metal, and which 

 he had heretofore so frequently described. By the use of this polish- 

 ing substance he had produced a plane surface of silver which, as far 

 as the photometric means he had within his reach would enable him to 

 measure the light before and after reflection, did not lose in that action 

 seven parts of the hundred, and which, tested in the manner which he 

 usually adopted, defined admirably. 



The Astronomer Royal begged to know how Lord Rosse secured the 

 plane form of the surface in grinding and polishing. The Earl of 

 Rosse replied that, as to the mode of grinding, it was that commonly 

 adopted for producing accurately flat surfaces. But the mode in which 

 he tested it was peculiar. It was this : a watch-dial was placed 

 before a good telescope ; and, as soon as the eye-piece was accurately 

 adjusted to the position of most distinct vision, the plane mirror was 

 placed in front of it, at an angle of 45, and the watch dial was moved 

 round by a simple contrivance to such a position as that its image 

 should very nearly occup}*- the place it had been just removed from. If 

 now the adjustment of the telescope for distinct vision remained 

 unchanged, the proper form had been attained ; but if by drawing out 

 the eye-piece more distinct vision was obtained, it was concluded it 

 had received a convex form ; if on pushing it further in it gave the 

 image more distinct, then it was concluded the mirror had received a 

 concave form. London Athen&um. 



ACTION OP SUNLIGHT UPON THE CONSTITUTION OF GLASS. 



PROF. FARADAY, at the British Association, exhibited a specimen of 

 dark glass, which had been sent to him, which had been acted on in a 

 curious manner by the solar beams concentrated at the eye-piece of a 

 telescope which magnified 100 times. It was the fourth or fifth dark 



