188 Royal Astronomical Society. 



flour, and of such a consistence that, at a temperature of 80° Fahr- 

 enheit, the nail can make an indentation in it. It is not to be un- 

 derstood that there is any particular virtue in the temperatures 50° 

 and 80°, but (for reasons to be hereafter given) it is necessary to 

 have two strata of different degrees of hardness (the harder being 

 the exterior) ; and the hardnesses defined by those two temperatures 

 having been used in many experiments, the other adjustments have 

 been determined, using these as bases ; and if these bases were now 

 changed, every other adjustment must be changed. 



The necessity for the different strata of resin is thus explained. 

 It is necessary that the polisher yield a little, else the polishing sur- 

 face could not be in contact with the mirror at all parts of its stroke 

 (the mirror being supposed parabolical), and this yielding is given 

 by the first or soft stratum. But it is also necessary that the stra- 

 tum next to the metal be hard ; for if, in passing across a scratch or 

 furrow, it were able to accommodate its form to that of the furrow, it 

 would round off and polish the edges of the furrow, and this would 

 very much injure the image of a bright object. 



The coating is heated to softness by the flame of a torch, and the 

 grinder is then lowered upon the mirror, and the coating takes the 

 proper form. The mirror is exposed to no danger of cracking from 

 this application. 



The powder used is the red oxide of iron, prepared by precipitating 

 (by means of ammonia) the black oxide of iron from a solution of 

 sulphate of iron, and then heating the black oxide in a furnace with 

 access of air. Lord Rosse finds that no other method of preparing 

 the red oxide is successful. No polishing-putty or other powder of 

 any kind is employed. 



The powder is moistened with water to a degree known by ex- 

 perience, and then, the counterpoise of the grinder being so much 

 increased that the remaining friction is enough to turn the grinder 

 only once for about sixteen revolutions of the mirror, the operation 

 goes on for about eight hours. It is essential, for reasons similar to 

 those lately mentioned, that the temperature of the air and the tem- 

 perature of the dew-point have nearly certain definite values (the 

 latter, that the water mixed with the powder may dry in the proper 

 degree) : if the external air is too dry, the air of the room is moist- 

 ened by a jet of steam ; if it is too damp, the polishing is not at- 

 tempted. 



After about eight hours the grinder is lifted and a fresh applica- 

 tion is made of powder mixed with " ammonia soap," a substance 

 formed by treating common soap with ammonia. The metal then 

 dries more rapidly, and the labour to the engine becomes much 

 greater : the work is continued till the surface is dry, or very nearly 

 dry ; the grinder is taken off, and the mirror is found finished, ha- 

 ving a parabolic figure, and a very high pohsh. 



It is to be remarked here that the smaller mirrors made by Lord 

 Rosse (3 feet diameter, about 25 feet focal length,) were tried before 

 they were used, by means of an object fixed something more than 

 50 feet above thepi, whose image accordingly is found at something 



