304 PHILOSOPHICAL TRANSACTIONS. [aNNO 17Q3. 



its importance deserves, I proceed to the description of one I have caused to be 

 made by a very able artist of this metropolis, Mr. Jesse Ramsden. 



Then follows a long and minute description of all the parts of this very com- 

 plex machine, illustrated by several copper-plates, not necessary to be here re- 

 peated. 



After which Sir G. adds : After the very rigorous examination the divisions of 

 these circles have now undergone, and from the general knowledge that I have 

 had opportunities to obtain of the state of practical astronomy in different coun- 

 tries ; and when I consider that the celebrated artist, the late Mr. John Bird, 

 seems to have admitted a probable discrepancy in the divisions of his 8 feet qua- 

 drants, amounting to * 3", I think I am entitled to believe that the accuracy of 

 these divisions under consideration is hardly to be equalled, and still less to be 

 excelled, by that of any astronomical instrument in Europe ; and from the un- 

 exampled diligence and care with which the skilful artist Mr. Matthew Berge, 

 workman to Mr. Ramsden, has executed them, I feel myself bound to bear this 

 testimony to his merit. 



It remains that I now say something of the power of the telescope; for it is to 

 little purpose that the divisions be accurate, or the levels sensible, unless the 

 force of the telescope be such as to correspond with the sensibility of the one, and 

 the accuracy of the other. The object-glass is a well corrected double achromatic, 

 whose joint focus is 65 inches, with an aperture of 4.2 inches. The telescope is 

 furnished with 2 sets of eye-glasses, one single, the other double ; of these latter 

 there are 6, of different magnifying powers, from 6o to 36o times; of the former 

 there are 5, with powers from 1 50 to 550. To these may be added a prism eye 

 tube, with a power of about 100, for objects near the zenith, or the pole, and 

 similar to the one described by General Roy, in the Philos. Trans, vol. 80, also 

 a tube with a divided eye glass micrometer ; see Philos. Trans, vol. 7g ; it has a 

 power of 80, but the images are not distinct, or equally bright, and the extent 

 of the scale is so small, not more than lO', that it is in truth but of little use. 

 The double eye tubes are composed of 2 eye-glasses, to enlarge the field and 

 render it more agreeable, both placed an the hither side of the cross wires, so 

 that they may at any time be changed, without deranging the wires. The lowest 

 of the compound eye tubes, with a power of about ()0, is what is generally used 

 for transits and polar di8tances.-|- For telescopical observations of the planets, 

 higher powers may be put on ; and of these, that of 400 seems to be near the 



* See Mr. Bird's method of constructing mural quadrants. London, 1768. — Orig. 



+ If, as has been generally imagined, an angle of 1' is about the smallest that is visible to the 

 naked eye, (Smith's Optics, § y?) with a power of 60 times, 1" will become visible ; and, in tliat 

 case, tlie power of this telescope will correspond with the levels, and the divisions, as was required 

 above. — Orig. 



