TEL 



formed in a position contrary to that which in formed by 

 the i' 1 Lastly, the' third 1< 



ire and the eve at a di-- u the 



r equal to iU local length, ti -veial 



pencils will rmrrire paiallcl to one another, and an 



of the object will thus be formed in the . 

 The ratio between the angles under which an objrct 

 woul'i :<\ thr naked eye, and that by wlucli it is 



orapouuded of the mtios O f the 

 lengths of thr several leu- if F be t!;>' 



TEL 



, 



length of thr object-lent, /'./". /'"' those of ll. 

 lenses, reckoned in onlrr towards the rye, the expression 



^-=7j, will denote the ma<-'iiifying power. 



Uut both the spherical aberration and the. chromatic 

 dispersion in such a telescope onsiderable; and 



before the invention of th chTOBMttia object-glass, Mr. 

 Dollond endeavoured to diminish the former by n c>c- 



. as to divide the 

 '.y equally bruvrcn them. 

 not no\v u Mr. Dollond suc- 



^d at length in constructing telescopes with four eye- 

 glasses, from which both distortion ami 10 re- 

 moved as much perhaps as a renu>\:il i- ; 



This is accomplished by placing i 

 yond the imairr formed by the object-glass, and at 

 tance from it less than the focal length ef th; 

 bv this disposition the ray* of mean refrangibihty in each 

 .1 which diverge* from the image are not, alter re- 

 fraction, parallel to one another, but noon with diminished 

 di>ci. i little way beyond the place where the 



of the oblique pencils cross the axis of the t*l< 

 is placed the ecoud eye-L'lasss whic.li is of Biich 

 length that the mean refrangible rays in each pencil 

 after passim; through it, meet in a point, and thus a .-eeoiii 

 image of the objrct is formed near the eve: the use of 

 lenses, therefore, is to cause the second image to 

 be formed by a gradual convergence of the rays in eacl 

 pencil. But the several pencils of rays are intercepted by 

 the third eye-glass (commonly called the field-glass ,, am 

 the second' image is thereby formed rather nearer to the 

 first than it would be without such field-lens: from this 

 image the rays in each pencil diverge, and by the refrac- 

 tive power of the fourth eye-glass they are made to enter 

 the eye in parallel directions: thus distinct vision of the 

 external object is obtained. The field-glass might have 

 been placed between the eye and the second image 

 a_s in the Huygenian eye-piece before described; but. tin 

 ahrnatinn arising from the spherical form of the glasses is 

 a little less by the construction just mentioned. 



Now, in each pencil, the red :uul violet rays which hat 

 formed by the object-glass, and 



which there eroded each other, go on from thriicr di- 

 verging from each other till, on the opposite side of the 

 . i' the telescope, they fall upon the (surface of the 

 'Her pasi Ii this lens, the 



\iolel ray, which is always more refi acted than the red 

 gradually converges towards the latter, and at length 

 n some pla. of that at. which the rays of 



rrfi action unite to form the second image. The 



to IU the third or field-:. 

 at ll 



of t! in parallel 



direr' ! a vii w :rly or 



who, ',ur. 



In formii - it may be ob- 



served that, they slm ill allow the incident 



and emergent pencils of ia\s to be ncaily equally i 

 to tli. ' ' and fourth eye- 



' side of that 



which is 's the 



Litter, and that of the ol! 



It bait been said above that, i 



speculum . of the t irpote 



of t! .lass in refractin 



us ; and the manner in which, r 



(I to the rye i 



diagram r< B longitudinal .ectioii 



through the axis XY of the instrument, which Lt supposed 



containing two lenses. MN is the a 1 ' the 



great speeulnni, which bus n rircnlai 'ti. nih, at 



fl is a small spendum. concave like tin 1 former, 



:.ice beiiiL' It is 



connected with the side of the tube AH l>\ the arm UK. 

 and is capable of brin^ moved in the direction of the 

 XY by means of the rod I'S : the latter passes through a 

 knob Q, which is fixed to the side of the tube, and works 

 in the knob H, which passes through an oM 

 in tho side of the tube, and is nttachcrt to the pan 

 the ami HK. This movement is given to thr small n; 

 in order that its locus may be made to coincide with the 

 place of the image formed by the iri-eat speculum; that 

 image bring at different d:- cord- 



ing to the distance of the > the oliservrr. 



Lei () be the upper part of an object, and let ON be the 

 direction of the rnys in a pencil of light diverging from < > : 

 the rays of this pencil will, after . -,.(, ,| ,,t N. 



to i,. which will bo the lower pnrt of the image ut. 

 From o the rays in the pencil diverge, and having fallen 

 upon the ratal] mirror at n, the\ <>m thence 



towards the eye-piece KF : having passed through the 

 orifice in/t, they fall on the lens at F. by which tli, 

 made to unite at ;<. where an in 



formed. From j> the rays in the same pencil again di- 

 vt-ige, and, falling on the lens at K, they are made to 



v ill parallel directions, so that the c\e is riiab. 

 obtain distinct \ision of the object in the - ion as 



if the latter were viewed bv the naked rjr. 



hring reflected at ;/. miirht with a due corn-ax 

 the smaller mirror have united, as at />', in front of the 

 great mirror, and the second image might have 



d at ii 1 !/ : in this case the ia\s in each pencil. 



ng one another, would have fallen in a divergent 

 state on the lens at F, and then, by the refract, 

 of both lenses they would have entered the eye in parallel 

 directions :-. Thr positions of thr ICIIM.-. at F and 



1 the curvatures of their surfaces, aie d ac- 



cording to the method of Huygens ; and the construction 

 differs in no respect from that which has been described in 

 speaking of the r\r-piecei of dioptric tc 



The magnifying power of a reflecting telescope of 



Gy 



kind is expressed by the. forum!: in which \z ii 



al length of the great speculum, Gy is the distance 

 of tin - -ulum fiom the image /.</, \;/ is the 



length of the second cM'-gla*s. and G- is the focal 1< 

 of the -mall speculum for parallel i: 



In ;dl mirror 



made . d it is pi. 1 .- 



alum MX bri'orr the ii: 

 formed; t! each i.'ncil conse<iurntly fall in a 



lirior, and. 

 .oriii the image i 

 refrae! t eye-c '.ass F. It : 



with rijual magnifying power, will be shorter 



than the Gregorian ' ioie than twice the 



focal 1> le small speculum ; and it may added that, 



grcr, the spherical aberration > d by 



V- two mirrors. 



.c one con 



Jin at. the boll . ill each pencil 



'it, thr ra- > a COIIM 



.h a Hi 



of the tube AH. 

 open end of the. tube. 



: iree. fixed in the side, 

 airror; thatU near the 



