optics. .303 



sents objects the most bright and distinct, and 

 bears the greatest aperture, and the shortest eye- 

 glass, without colouring or dimness, is the best 

 glass. 



The Reflecting Telescope. — The great inconve- 

 nience attending the management of long tele- 

 scopes of the refracting kind has brought them too 

 much into disuse since the reflecting telescope 

 was invented ; for one of this sort, six feet in 

 length, magnifies as much as one of the other at 

 an hundred. It was invented by Sir Isaac Newton ; 

 but has received considerable improvements since 

 his time ; and is now generally constructed in the 

 following manner, which was first proposed by Dr. 

 Gregory. 



At the bottom of the great tube TTTT 

 (Plate Y]. fig. 1.), is placed the large concave 

 mirror DUVF, whose principal focus is at M j 

 and in its middle is a round hole, P, opposite to 

 which is placed the small mirror L, concave toward 

 the great one, and so fixed to a strong wire M, that 

 it may be moved farther from the great mirror, or 

 nearer to it, by means of a long screw on the out- 

 side of the tube, keeping its axis still in the same 

 line, P m n, with that of the great one. Now, 

 since in viewing a very remote object, we can 

 scarcely see a point of it but what is at least as 

 broad as the great mirror, we may consider the rays 

 of each pencil, which flow from every point of the 

 object, to be parallel to each other, and to cover 

 the whole reflecting surface D U V F. But, to 

 avoid confusion in the figure, we shall only draw 

 two rays of a pencil flowing from each extremity 

 of the object into the great tube, and trace their 

 progress through all their reflections and refrac- 



