but above that exhibits a cross formed by the diffraction at 

 the joints ; and in unsteady states of the air exhibits the six- 

 teen divisions of the great mirror on the star's disk. By dimi- 

 nishing the number and size of the joints it is found, that 

 these inconveniences can be diminished, so as to be scarcely 

 perceptible ; and in all probability this is the process by 

 which the remotest limits of telescopic vision veill ultimately 

 be attained. It is, however, not necessary 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 effec- 

 tive on a much larger scale. 



But however difficult it may be to obtain the rough spe- 

 culum of large dimensions, it is still more so to give it a 

 proper figure, combined with that brilliant polish which is 

 technically called black, because it reflects no light out of 

 the plane of incidence. In such mirrors 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 por- 

 tions of its pitch covering, a parabolic figure is obtained. 

 For sizes above nine inches diameter, the work must be per- 

 formed by machinery ; but in all which Dr. R. 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 Herschel is believed to have made important 



* Dr. R. had the good fortune to see this at Slough, in 1830, while at work 

 on a twenty- feet mirror. 



