▼OL. XLVIII.] PHILOSOPHICAL TRANSACTIONS. 301 



through w, let a chord line be drawn parallel to the diameter dm, viz. bg, hp. 

 Through the said chord lines bg and hp, and also through the diameter dm, 

 divide the lens into 4 parts. 



Fig. 3, Let the straight edge of the frustum bvgq, in the preceding figure, be 

 cemented fast to that of the similar frustum hwpf of the same lens, as they ap- 

 pear in this fig. 3. Having then with barm fastened a white paper all over both 

 sides of the lens, he made for trial (which he did, not only to secure the ce- 

 mented joint from breaking, but to prevent the injury which the polish might 

 receive in cutting and grinding the edges) he described a circle qmnf on the 

 centre c. fit for the tube he had to put in it: and having made it round, and 

 washed it clean, after the edges were ground true, that nothing sandy might 

 hurt the polish, he soaked it in clean water, till he could easily take off the 

 paper. This model, made of a spectacle glass about 1 2 or 1 3 inches focus, gave 

 him encouragement to try the following one, which he thought better. 



Fig. 4, Mr. S. made his second model of the 2 middle frustums mcdhwp, 

 mcdbvg, of the lens in fig. 2, by cementing their edges, hwp, bvg, together, as 

 they are placed in the present fig. 4, so the pole c of each part must consequently 

 be half an inch, supposing its focal length is about 104 inches, from the middle 

 where c stood in fig. 2, viz. the pole of one frustum where v, and of the other 

 where w now stands. He left open at each pole a semicircular aperture rwq, svt, 

 about ^ of an inch diameter, and covered all the rest of the circle axlkzo, to 

 which he had cut it fit for the tube. The focus of the lens he made it of, was 

 about 3 feet. 



Note, The rays of red light in the two solar images will be next to each other 

 in both these models, which, he thinks, will render the sun's disk more easy to 

 be observed than the violet ones. This he mentions, because the glasses in these 

 two sorts are somewhat prismatical, but mostly those of the first model, which 

 could therefore bear no great charge. Also the frustum on the right hand of the 

 first model renders the solar image at the focus on the left, and that on the left 

 hand renders it on the right; but it is not so with the second model, or with the 

 next contrivance, which is the best, if well made. 



Fig. 5, In this, the greatest difficulty consists in getting two well-centred ob- 

 ject-glasses, whose focal lengths are equal; for it is necessary they should be so, 

 because they are to be combined with the same convex eye- lens, common to 

 them, at the same distance, ab is the diameter of a plain brass plate, which 

 may be 2-i inches broad, or somewhat less; two short equal cylindric brass tubes 

 mn, rs, must be fastened on it, with their centre pc, equidistant from the centre 

 1 of the plate, and distant 1 inch from each other in the diameter ab, as the 

 figure shows. In the tubes must be put two equal object-glasses of the focal 

 length of 104-rV inches, or rather somewhat less, as aforesaid. Through the 



VOL. X. 3 A 



