586 Royal Irish Academy. 



divisions of the great mirror on the star's disc. By diminishing the 

 number and size of the joints it is found that these inconveni- 

 ences can be diminished, so as to he scarcely perceptible ; and in all 

 probability this is the process by which the remotest limits of tele- 

 scopic vision will ultimately be attained. It is, however, not neces- 

 sary for instruments of even greater dimensions than this, since Lord 

 Oxmantown has succeeded, by a contrivance as simple as ingenious, 

 in casting at the first attempt a solid mirror of the same size ; and 

 there is no reason to suppose that it will be less effective on a much 

 larger scale. 



But however difficult it may be to obtain the rough speculum of 

 large dimensions, it is still more so to give it a proper figure, com- 

 bined with that brilliant polish which is technically called black, be- 

 cause it reflects no light out of the plane of incidence. In such mir- 

 rors as can be wrought by hand, they are worked by short cross strokes 

 on the polisher, and at the same time have a slow rotation relative 

 to it. This might be expected to produce merely a spherical figure ; 

 but by varying the length of the stroke, by circular movement, elliptic 

 figure of the polisher, or removing portions of its pitch covering, a 

 parabolic figure is obtained. For sizes above nine inches diameter, 

 the work must be performed by machinery : but in all which Dr. Ro- 

 binson has seen (the most remarkable of which are those of Sir 

 William Herschel* and Mr. Grubb), the cross stroke is given by a lever 

 moved by hand : and it is supposed that perfect results cannot be 

 obtained but by the feeling of the polisher's action. Sir John Her- 

 schel is believed to have made important additions to his father's 

 apparatus ; and it is to 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 trans- 

 verse and circular grooves into portions 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 direction, while it is also im- 

 pelled with two rectangular movements. 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 com- 

 plete. By an induction from experiments on mirrors from six to 

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

 transverse movements be ^rd and y^th of the aperture, and their 

 times be to its period of rotation as 1 and T8 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. 



* Dr. Robinson had the good fortune to see this at Slough, in 1830, 

 while at work on a twenty-foot mirror. 



