364 PHILOSOPHICAL TRANSACTIONS. [aNNO 1753. 



This telescope may be finished with a small elliptical specillum of black glass, 

 ground plain on its reflecting surface, and a convex eye-lens, like that described 

 by J. Hadley, Esq. p. r. s. in Phil. Trans. N° 376. A micrometer may be con- 

 trived for it at the common focus, near the eye-lens. 



Such a double-object speculum would be capable of a vast improvement, by 

 combining it with a concave specillum, ^vhich would reflect the images through a 

 hole in the centre c of the said speculum to fall on a convex eye-lens, after the 

 manner of our new sort of reflecting telescopes, were it not for the difficulty of 

 adapting such a micrometer to it as would exactly measure minutes and seconds ; 

 for the eye-glasses of such having usually a pretty large focal length, would bear 

 much larger divisions on a micrometer, than Mr. Hadley's with a small eye- 

 glass can do, though their charges should be equal, or that of the former did 

 exceed. 



Finding that large object-glasses for telescopes are not commonly well cen- 

 tered, with their poles in the very middle of them, gives the following a rule for 

 centering optic-glasses ; which may be very ready for a glass-grinder's use, and 

 soon try whether a convex lens is well centered. 



Fig. 8, represents a round plate of brass, conveniently thick, and well har- 

 dened by hammering, having many notches round it, one a little wider than 

 that which is next to it, and numbered 1, 2, 3, &c. in their proper order, each 

 of them wider at the bottom than at the entrance. He fitted such a notch to 

 the thickest side of one of the glasses he had received from London, so as the 

 edge entered it but a little way, not half its depth ; but, on trying the opposite 

 side, it went in, the whole depth, and would have gone deeper, if the notch 

 had been so cut : he then ground the lens narrower on that side which was 

 thinnest, till he found it was at that place as thick as where he first tried it in 

 the notch. After this manner he reduced the glass to an equal thickness on its 

 4 quarters, and then ground off from other places what was needful to bring it 

 circular. He also took care, when he tried it in the notch, that the lens should 

 not be warmer on the one side than on the other by grinding, but stopped till it 

 was thoroughly cold ; and was also careful not to thrust it in harder on the one 

 side than on the opposite side ; for he could plainly observe a diflerence after- 

 ward, if he neglected to mind both these circumstances, or indeed either of them.* 



XXVII. Of a Contrivance for Measuring Small Angles. By Mr. John Dol- 



lond. p. 178. 



Let an object-glass, of any convenient focal length (being truly ground and 



* Dr. Smith, in his Complete System of Optics, published in 1738, has described a very accurate 

 and ready method of centering object-glasses, which was always used by the late Mr. George Graham, 

 from whom the Doctor had it. — Orig. 



