TELESCOPE. 



excelled in this particular, and in other praftical niceties, 

 would initiate their fucceflbrs in the fccrets that promoted 

 their excellence, that pofterity may benefit from their fuc- 

 cefsful labours ; which could not have been Mr. Short's 

 wifli, when he deliberately provided for the deftruftion of 

 his beil tools, after he no longer wanted them. 



The circumftances that led to the confti-uftion of a rc- 

 flefting telefcope did not arife out of chance, as is fuppofed 

 to have been the cafe with tlie dioptric, but out of the diffi- 

 culty of avoiding the indiJlinBncfs produced by aberrations 

 of both kinds ; and the tirft arrangement that would pro- 

 bably occur, would be that of a fpeculum oppofed to the 

 eye of the obferver, whofe head in that cafe would intercept 

 tlie incident rays, and prevent their falling on tiie fpeculum, 

 unkfs it were made of an unmanageable diameter. To avoid 

 this inconvenience, Gregory, who was the firft to undertake 

 the arduous taflc of a new conftruftion, devifed the ex- 

 pedient of opening a hole in the centre of tlie large fpecu- 

 lum, fufficiently large to adinit of tlie rays that came re- 

 flefted a fecond time from a fmaller fpeculum without a 

 central perforation : it would naturally occur to him, that 

 if this fecond fpeculum was not larger in diameter than the 

 central hole of the large fpeculum, no incident light would 

 be intercepted by it, when the rays came parallel, but what 

 would have pafled through the central hole of the fpeculum. 

 This confideration brought the eye to the fame end of the 

 tube in which the large fpeculum was placed, and thus freed 

 the aperture from all obftacles to the free admiffion of light ; 

 but whether the fmall fpeculum at firft tried was a plain one, 

 or concave, is not of importance to afcertain ; it is fufficient 

 for our purpofe to know, that a concave one was ultimately 

 adopted, and probably from the property which it poflefles 

 of varying the magnifying power to a confiderable extent. 



We have already faid that Gregory's conftruAion of the 

 reflefting telefcope is the moll ancient, and indeed continues 

 to be the moft common, even at this time, on account of 

 the convenience attending its ufe, and therefore we will be- 

 gin with an explanation of its theory. PiauXXVll.Jig. i. 

 yljlronomical Iajlrur>icnts, reprefents a fcftion of this mftru- 

 ment as it was originally made, ixiAjig. 2. is a reprefentation 

 agreeably to its improved modern conftruAion; in both which 

 we (hall ufe the ffame letters of reference to the correfponding 

 parts. A B C D, in each figure, denote the tube of wood 

 or brafs in which tvfo concave fpecula are contained ; the 

 large one, B D, is perforated at the centre, and placed con- 

 tiguous to the interior end of the tube, but in luch a way 

 as to have a little play when preffed by a circular fpring be- 

 hind it ; E F is the fmall fpeculum, which is sf (hotter radius 

 than the fpeculum B D, and has its centre placed exaftly in 

 the centre of the tube oppofite the central aperture in the 

 large fpeculum, and is fo adjufted by the fcrews behind it, 

 that the image of the large fpeculum forms a concentric 

 circle on its reflefting furface, when viewed by an eye fitu- 

 ated in the central hole of the large fpeculum. In this in- 

 ftrument, as in the refradting telefcope, it will be moft con- 

 venient to defcribe firft the formation of the primary image 

 of a diftant objeft in the body of the tube, and then the 

 microfcopic means applied for nrr;dering this image vifible in 

 an apparently magnified ftate ; for in truth there is aftually 

 a compound refleHlng micro/cope made ufe of as a confti- 

 tuent part of this inftrument, in hke manner as the terref- 

 trial tube of a refrafting telefcope of the beft achromatic 

 conftruftion, is in itfelf a compound refratilng micro/cope. 

 In the firft place, agreeably to the laws of catoptrics, which 

 we have explained, if we confider a b and c d two rays of 

 light coming from the centre of a diftant arrow in a ftate of 

 tiivergence approaching to parallelifm, and impinging on the 



large fpeculum at the points k and d near the remote edges 

 of the fpeculum, and at equal diftances from Us axis, they 

 will be refleftcd inwardly fo as to meet at the point e, in the 

 common axis of both the fpecula, and will form the image 

 of the central point of the arrow ; and in hke manner, any 

 number of rays proceeding from the oppoGte ends of the 

 faid arrow may be conceived to fall on the fpeculum, and to 

 be reflcaed to the points h and i, and to all the int.rnicdialc 

 pomts, fo as to form a perfect image hei in an inverted po- 

 fition, becaufo the r.iys which enter the tube from the right- 

 hand end of the arrow, will after refledion crofs the axis, 

 and form the left-hand end of the image, and -vice verfd. 

 When an image is thus formed, if it could be viewed, under 

 fufficiently favourable circumftances, by an eye placed in 

 the vertex or central aperture of the large fpeculum, it 

 would fubtend the fame angle as the objea itfell fecn from 

 the fame fituation, as we have already demonftraled ; and 

 therefore the length of the image will bear the fame propor- 

 tion to the length of the objed which it reprefents, as its 

 diftance from tlie eye, or vertex, is to that of the objea ; 

 fo that the longer the radius of the fpeculum which forms 

 the image, the more diftant, and confequently the longer 

 will this image be, as compared with the objeft ; and lor 

 the fame realon, the nearer the objeft, the longer will its 

 image be, until the fituation is at the centre of concavity of 

 tlie fpeculum, where the objeft and image will coincide, and 

 appear of like magnitude, but in contrary pofitions. 



This formation of the primary image being underftood, 

 we muft in the next place confider it as a real microfcopic 

 objeft, placed fomewhcre between the face of the large 

 fpeculum, and its centre of concavity, which fituation will 

 always depend on the diftance of the real objeft itfelf, or, 

 which is tile fame thing, on the degree of divergence of the 

 incident rays coming from the objeft. Now if the fmall 

 fpecuUm were fo placed as to have this primaiy image, or 

 microfcopic objeft, in its folar focal point, the rays coming 

 from it would be reflefted towards the large fpeculum in a 

 parallel ftate ; and palling through the central opening of the 

 large fpeculum, would never converge fo as to form a 

 fecondary image, in which cafe the conjugate focus would 

 be faid to be infinite : and if the faid primary image were 

 nearer to the fmall fpeculum than its folar focus, the re- 

 flefted rays would diverge fo as not to reach the central 

 hole of the large fpeculum at all ; but if the diftance of the 

 primary image h e i exceeds the folar focus of the fmall 

 fpeculum E F, which is at the point /, then the reflefted 

 rays coming from the primary image will converge to a 

 conjugate focus fomewhere in the axis, and form ajeconjary 

 image, the magnitude of which will incrcafe with its diftance 

 from the primary image, which we now confider as a real 

 microfcopic objeft. The place where this fecondary image 

 will fall, will depend on the diftance of the primary image 

 from the folar focus of the fmall fpeculum; and a fmall 

 change of this diftance will caufe a great correfponding 

 change in the place of the fecondary image, or conjugate 

 focus ; fo that an adjuftment for a fmall forward and back- 

 ward motion of the fmall fpeculum, by means of a fcrew 

 at the end of a long rod placed parallel to the tube, and 

 reaching to the eye-end, will fufficc for regulating the place 

 where the fecondary image ftiall moft conveniently fall to 

 be viewed by an eye-glafs. The fecondary image has its 

 pofition reverfed, as it regards the primary one, and is there- 

 fore in the fame pofition as the objeft itfelf, or what is 

 ufually called enS, in oppofition to inxerud. This fecondary 

 image was originaDy made to fall within the tube, as at k I, 

 m the focus of the eye-glafs G H, through which it may 

 be viewed by a fmall hole at I, where the vifual angle 



GIH 



