5S4 Royal Astronomical Society. 



and then adding a small quantity of white arsenic. It is well known 

 that very great importance was attached to the admixture of arsenic 

 by the principal constructors of reflecting telescopes in the last cen- 

 tury. Lord Rosse has uniformly stated it as the result of his expe- 

 rience, that when the copper and tin are properly proportioned, 

 nothing is gained by adding arsenic ; Mr. Lassell is equally confident 

 that the brightness of the metal is much improved by it. It would 

 be idle to express an opinion on this point without comparing speci- 

 mens of the two kinds of speculum-metal side by side ; and the 

 Astronomer Royal only undertook to say that both bear a very high 

 polish. The Astronomer Royal believed that the operation of an- 

 nealing Mr. Lassell's mirrors was nearly similar to that for Lord 

 Rosse's, although the trouble and risk of the process are of course 

 materially diminished by the much smaller dimensions of the spe- 

 culum. 



The next step in Lord Rosse's operations is to turn (by a grinding 

 process, with emery as the abrading powder) the edge of the mirror, 

 the mirror being placed with its broad surface horizontal, with its 

 lower surface and about one-third of its depth immersed in water, 

 and being turned horizontally by a vertical spindle passing through 

 a stufling-box in the bottom of the water vessel. The purpose of 

 this turning is, to make a nearly air-tight fitting for a covering which 

 is to be applied to the mirror when it is not in use, and with which 

 is connected a box of quicklime, for the desiccation of the air in 

 contact with the mirror ; and the object of turning it in water is, to 

 keep all parts of the speculum in the same temperature, a caution 

 which is necessary in every part of the operations. This caution is 

 probably requisite only for specula as large as those of Lord Rosse. 

 For these it was found by Lord Rosse, that if the iron grinder (to 

 be mentioned presently) be washed with warm water, and be then 

 applied to the speculum, the metal almost infallibly cracks. A sur- 

 face of warm pitch may, however, be applied without producing the 

 same bad effect. 



II. The subject next worthy of attention, rather for the similarity 

 of the methods of the two constructors than for their dissimilarity, 

 is the method of mounting the mirrors. And this seems to be a 

 proper place for mentioning the mounting, because it is indispensable 

 (at least with Lord Rosse's specula) that the mirror be ground and 

 polished on the very same supports, applied in the very same manner, 

 as when the mirror is in use in the telescope. 



Tlie special object of Lord Rosse's support is this : supposing the 

 surface of the speculum to be divided into any number of equal por- 

 tions, each of a form not particularly elongated in any direction, then 

 the supports are to be so arranged that every one of these portions 

 shall necessarily sustain the same upward pressure, acting at its 

 centre of gravity. At the same time it is necessary, for definiteness 

 of support, that the ultimate support upon the fixed frame be upon 

 three points. These objects are thus attained : the surface of the 

 mirror is divided into twenty-seven equal portions, and these are 

 grouped into nine groups, of three in each ; to each portion is at- 



