20 



ON THE CONSTRUCTION AND USE OF 



The machine, which is a simplification of Lord Rosse's, was intended to give 

 spiral strokes. It differed from the original, however, in demanding a changeable 

 stroke, and in the absence of the lateral motion. In another most essential feature 

 it varied from both that and Mr. Lassell's, the mirror was always uppermost while 

 polishing, and being uncounterpoised escaped to as great an extent as possible from 

 the effects of irregular pressure. To any one who has studied the deformations of 

 a reflecting surface, and knows how troublesome it is to support a mirror properly, 

 the advantage is apparent. 



Fig. 21. 



PoUaliiug Macliiue. 



The construction is as follows: A stout vertical shaft, a, Fig. 21, carries at its 

 top a circular table 6, upon which the polisher c is screwed. Below a band-wheel d 

 is fixed. Above the table, at a distance of four inches, a horizontal bar e is arranged, 

 so as to move back and forward in the direction of its length, and to carry with it 

 by means of a screw Z, the mirror ?», and its iron back or chuck n. The bar is 

 moved by a connecting rod /, attached to it at one end, and at the other to a pin g 



Fig. 22. 



The Foot Power. 



moving a slot. This slot is in a crank A, carried by a vertical shaft *, near the 

 former one a. The band-wheel k is connected with the foot power. Fig. 22. The 



