VOL. VI.] PHILOSOPHICAL TRANSACTIONS. 6Q1 



pocket volume, all the parts of the Cartesian philosophy, to facilitate the study 

 of the same to such as desire to instruct themselves in it. 



III. An Essay to the Advancement of Music. By Tho. Salmon, M.A. 

 London 1672, in 8vo. 



The design of this essay is, to advance music by casting away the perplexity 

 of different cliffs, and uniting all sorts of music, lute, viol, violin, organ, harp- 

 sichord, voice, &c. in one universal character. 



END OF VOLUME SIXTH OP THE ORIGINAL. 



An Account of a New Catadioptrical Telescope, invented hy Air. New- 

 ton, F. R. S. and Professor of Mathematics in the University of Cam-- 

 bridge.* iV" 81, p. 4004. Vol. FIL 



This excellent mathematician having given us, in the Transactions of Feb. 

 last, an account of the cause which induced him to think on reflecting tele- 

 scopes instead of refracting ones, has thereupon presented the world with an 

 essay of what may be performed by such telescopes; by which it is found, that 

 telescopical tubes may be considerably shortened without prejudice to their 

 magnifying effect. 



This new instrument is composed of two metalline specula, the one concave, 

 (instead of an object glass) the other plain; and also of a small plano-convex 

 eye-glass. By fig. 1. of pi. 15, the structure of it may be easily imagined, viz. 

 that the tube of this telescope is open at the end which respects the object ; 

 that the oiher end is close, where the said concave is laid, and that near the 

 open end there is a flat oval speculum, made as small as may be, the less to ob- 

 struct the entrance of the rays of light, and inclined towards the upper part of 

 the tube, where is a little hole furnished with the said eye-glass. So that the 

 rays coming from the object, first fall on the concave placed at the bottom of 

 the tube, and are thence reflected toward the other end of it, where they meet 



* In this paper we have the description of the first reflecting telescope that was ever made, as far 

 as we know. The idea had indeed been mentioned a few years before, viz. by Mersenne, in a letter 

 to Descartes, who did not approve of itj and again by James Gregory, in his Optica Promota, who 

 endeavoured in vain to carry the idea into execution. Those attempts however were suggested by a 

 motive far inferior to that of Newton, being intended, besides shortening the telescope, to avoid the 

 errors arising from the figure of the lenses ; whereas that of our author was to obviate the error and 

 inconvenience of the coloured images, and of the unequal refi^ction of the rays of light; a splendid 

 discovery, which had but just before been made by himself. 



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