for the Advancement of Science. 



419 



If 



I 



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 suc- 

 ceeded in producing a polishing surface which seemed fully to answer 

 the purpose by exposing spirits of turpentine to the continued action of 

 air, or by dissolving a proper quantity of resin in the spirits of turpen- 

 tine, and by means of this varnish applying the rouge to the same de- 

 scription of polishing substance which he used in polishing the specu- 

 lum metal, and which he had heretofore so frequently described. By 

 the use of this polishing 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 lin-ht before and after reflexion, did 

 not lose \n 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 be- 

 fore a good telescope, and as soon as the eye-piece was accurately ad- 

 justed to the position of most distinct vision, the plane mirror was pla- 

 ced 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 occupy the place it had been just removed from. If 

 now the adjustment of the telescope for distinct vision remained un- 

 changed, 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 farther in it gave the image 

 niore distinct, then it was concluded the mirror had received a concave 

 form. Prof. Chevallier wished to know whether Lord Rosse intended 

 to form the great speculum similarly of silver. The Earl of Rosse 

 replied that he had at present no expectation of doing so. That it was 

 a very different matter to grind and polish a speculum of a few square 

 inches surface — which could be done by a small machine worked by 

 hand, and from which to the eye-piece the light had to travel but about 

 three feet — to executing the same operations over the surface of a 

 speculum six feet in diameter, and from which the light after reflexion 

 had to travel a distance of fifty-three feel. For Newton, with his usual 

 sagacity, had long since shown that any error in the form of the object 

 speculum of a reflector was a much more serious injury to the perform- 

 ance of the instrument than an equal error would be in the plane spec- 

 ulum 



•and that for the identical reason he had just pointed out. 



2. 



Moist 



4 



of determining the quanlily of Hygrometric 



In the absence of Dr. Andrews, Prof. Stevelly made this commu- 

 nication. Dr. Andrews had found on trial that several powders when 

 well dried, would rapidly, effectually and completely take up the mois- 

 ture of damp air passed through them, as effctuaily as the fused chlo- 

 rid of calcium, which is too troublesome in the making, preserving 



