additions to his father's apparatus; and it is t > be hoped he 

 will soon redeem his promise (Mem. R. Ast. Soc. vol. vi.) 

 of publishing his improvements. 



Lord Oxmantown has in many respects deviated from 

 the usual process. His polisher, of the mirror's diameter, 

 intersected by transverse and circular grooves, into por- 

 tions not exceeding half an inch of surface, is coated, first, 

 with a thin layer of the common optical pitch, and then with 

 a much harder compound. It is worked on the mirror, and 

 counterpoised so that but little of its weight bears ; but the 

 want of pressure is compensated by a long and rapid stroke. 

 The mirror revolves slowly in a cistern of water, maintained 

 at a uniform temperature, to prevent the extrication of heat 

 by friction. The polisher moves slowly in the same direc- 

 tion, while it is also impelled with two rectangular move- 

 ments. The machine is driven by steam, and requires no 

 superintendence, except to supply occasionally a little water 

 to the polisher, and to watch when the polish is complete. 

 By an induction from experiments on mirrors from six to 

 thirty-six inches aperture it was found, that if the magni- 

 tudes of the transverse movements be i and yg^ of the aper- 

 tui'e, and their times be to its period of rotation as 1 and 

 1*8 to 37, the figure will be parabolic : but to combine with 

 this the highest degree of lustre, it is found necessary to 

 apply, towards the close, a solution of soap in liquid ammonia, 

 which seems to exert a specific action. 



The certainty of the process is such, that the solid 

 mirror of thirty-six inches aperture, after being scratched all 

 all over its surface with coarse putty, was, in Dr. R.'s pre- 

 sence, perfectly polished in about six hours, and was placed 

 in its tube for examination, without any previous trial as to 

 quality. 



Lord Oxmantown has preferred the Newtonian to the 

 Herschelian form, and, in Dr. R.'s opinion, with good 



