Chap. 8.] . RejleBing telefcope. 25$ 



difcover the magnifying power, it is neceffary on 7 .y to 

 compare together the angles under which the object is 

 feen through the eye-glafs, and that under which it 

 would 'have been feen by the naked eye. 



Inftead of lenfes only, for reafons hereafter to be 

 mentioned, a combination has been formed of refiect- 

 ing furfaces and lenfes, and from the names of the 

 inventors, thofe of the greateft ufe are now called the 

 Newtonian, Gregorian *, and Herfchelian telefcopes. 

 The reflecting telefcope on the Gregorian principle, 

 which is the mod common, as it is found to be the moft 

 convenient, is conftructed in the following manner. 



At the bottom of the great tube (PlateXXII. fig. 46.) 

 T T T T is placed a large concave mirror D U V F, 

 whole principal focus is at *, and in the middle of 

 this mirror is a round hole P, oppofite to which is 

 placed the fmall mirror L, concave toward the great 

 one, and fo fixed to a ftrong wire M, that it may be 

 removed further from the great mirror, or nearer to 

 it, by means of a long fcrew in the iniide of the tube, 

 keeping its axis ftill in the fame line P m n with that of 

 the great one. Now, fince in viewing a very remote 

 object, we can fcarcely fee a point of it, but what is, 

 at lead, as broad as the great mirror, we may con- 

 fider 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 furface D U V F. But 

 to avoid confufion in the figure, we (hall only draw 



* The difference between the Newtonian and Gregorian tele- 

 fcope is, that in the former the fpe&ator looks in at the fide 

 through an aperture upon a plane mirror, by which the rays re- 

 flecled from the concave mirror are reflected to the eye glafs, 

 whereas in the latter, the reader will fee that he looks through 

 the common eye-glafs, which is in general more convenient, and 

 therefore that is the telefcope which is. now in the moil univerfal 

 repute. 



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