Dr. Draper's Telescope. 371 



that if this peculiar form could be produced at will, and to an 

 adequate degree, it would render the Lemairean, or front view 

 telescope, perfect. Dr. D., however, found that the image was 

 never quite as fine as in the usual kind of mirrors. A letter 

 of Maskelyne subsequently came under his notice, in which 

 he describes a very great improvement effected in a 6-foot 

 reflector by Short, by inclining the large speculum 2i°, and 

 remarks very reasonably that " probably it will be found that 

 this circumstance is by no means peculiar to this telescope • " 

 a hint which may be worthy of the consideration of the pos- 

 sessors of reflectors. Such surfaces require to be reground, 

 or ' ' re-fined," i. e., finished with the finest emery, to get rid of 

 this obliquity, as repolishing, though occasionally successful in 

 a few minutes, will not always effect it ■ the attempt failed in 

 one instance though continued for 13-L hours. 



The modes of forming the requisite tools, of preparing* 

 emery, of grinding, polishing, testing (by Foucault's mode), 

 and silvering the surface, are somewhat too technical to find a 

 place here, but some interesting facts are worthy of being 

 referred to. Such is the effect produced by the removal from 

 a cast-iron tool, 15i inches in diameter, divided into f inch 

 squares, like a chessboard, of every alternate square by an 

 acid. Though the corrosion extended only to a very slight 

 depth, it flattened the curvature of the tool 7-f- inches. ' c This 

 shows what a state of tension and compression there must be 

 in such a mass, when the removal of a film of metal, T '-oth of an 

 inch thick, here and there, from one surface, causes so great a 

 change." Another important remark is, how injurious an 

 atmospheric disturbance is set up by the intermixture of cur- 

 rents of warm air from the observer's person with the rays 

 falling on or reflected from the mirror — an observation which 

 I made many years ago, and which any one may test by direct- 

 ing a Newtonian to any bright object, and placing one hand 

 beneath the aperture, while an eye-piece held in the other 

 hand, and applied to the eye, is carried back a considerable 

 distance, so as to obtain a very long focus, and render the 

 ascending currents more visible. It has not, I believe, been 

 generally remarked how prejudicial an effect this must have 

 on definition in the front-view reflector, and it would be a 

 worthy object of attention to remove the evil by the inter- 

 position of some non-conducting shield. 



A full trial was given to no less than seven machines, on 

 the principles employed by Lord Rosse and Lassell, with 

 modifications of his own. The prime mover, called the " foot- 

 power," was a very ingenious contrivance, in which very 

 little force is lost in overcoming friction, and which is fre- 

 quently employed in America for dairy use. v Dr, D. himself 

 VOL. VII. — no. v. BB 



