TELESCOPE. 



placed oppofite the central hole of tlie larcre one, in fuch an 

 adjuftable manner, that the rays, after a fecond rcflcftion, 

 crofs one another, and come to the eye-glafs in fuch a way, 

 that an ereS pifture of the objeft, or rather of the image of 

 the object, is formed on the retina of the eye. In this con- 

 ftruftion, it has been fuppofed that the figure of tlie large 

 concave fpeculum ought to be truly parabolic, bccaufc this 

 is the figure recommended by Newton for his conllruilion ; 

 but this conclufion is erroneous ; for it is the joint effeft of 

 both the fpecula that muil be adverted to in tlicirrefpcftive 

 figures, fo that the rays may come without aberration to the 

 eye-glafs after bolh reflexions ; and in order to produce this 

 joint effeft, the curve of the large fpeculum muft be fome- 

 what more than parabolic, ■u/'z. approaching to hyperbolic, 

 becaufe the finall fpeculum is alfo concave, and has its fepa- 

 rate aberration. 



In the Newtonian conftruAion, the large fpeculum is, or 

 ought to be, truly parabolic, and the fmall one plane, fet 

 diagonally at an angle of 45° ; fo that the rays, after the 

 fecond refleftion, come to the eye-tube on the JtJe of the 

 large tube, and near its aperture : the rays do not crofs 

 here, but come to a focus at the eye-glafs, where the objeft 

 is reprefented inverted and well defined, as well as bright ; 

 for when the rays fall obliquely on the fmall refleftor, they 

 are almoil all reflefted without difperfion, which is an ad- 

 vantage that this conftruftion has over the Gregorian. 

 When the Newtonian telefcope was propofed to Huygens, 

 he had the candour to acknowledge, which proved to be the 

 faft, that there would not be that limit to the apertnre of a 

 refleftor, that is prefcribed by natural neceffity to that of a 

 refraftor, and that the power as well as light may be made 

 far to exceed thofe of the latter. 



The next conftruftion of a reflefting telefcope was that 

 of Caflegrain, defcribed in the Philofophical Tranfaftions 

 of the year 1672. This differs from the Gregorian only in 

 this particular, that the fmall fpeculum is convex, and the 

 focus of the large or concave one may be longer than is re- 

 quired in the other, for the fame length of tube ; the rays 

 do not crojs after the fecond refleftion, and confequently the 

 objeft is feen inverted, as in the Newtonian : but here the 

 curve of the large fpeculum is lefs than parabolic, in order 

 that the joint effedl of both the refleftions may be an ex- 

 emption from aberration. Tliis adjuftment of the figures of 

 the metallic furfaces is beft underilood and accomplifhed by 

 the firft-rate opticians, and is but little known to mere 

 theorifts. 



Of the Herfchelian telefcope we (hall only fay, in this 

 place, that it differs from the Newtonian in no other refpeft, 

 except in its fize and powers, and that the fecond refleftor 

 is difpenfed with, the length of the tube being equal to the 

 focal diftance of the large fpeculum, and the head of the ob- 

 ferver being confequently placed at the upper end or aper- 

 ture of the tube ; fo that, in this conllruftion, as little light 

 as poffible is left from the fingle refleftion, the principal lofs 

 being that which is intercepted, on its entrance into the tube, 

 by the head of the obi'ervcr. The parabolic curve for the 

 face of the fpeculum is equally proper for the Herfchelian as 

 for the Newtonian telefcope. 



From thefe fhort hiftorical notices it will be feen, that 

 Merfenne firft fuggefted the hint for conftrufting a reflefting 

 telefcope, which muft have been before the year 1651, when 

 his Catoptrics were printed ; or, according to Defcartes' 

 third and twenty-ninth letters, written in 1639, though not 

 pubhfhed till 1666, before thefe letters were written. Gre- 

 gory, who might or might not have feen Merfenne's fug- 

 s;eflion, publifhed an account of his conflruclion in his 

 " Optica Promota," in the year 1663 ; but as he was not 



a fkilful mechanic himfelf, it is undcrllood that his telefcope 

 was but an indifferent one, and that the theory of his con- 

 ftruftion was not completely reahzed to his wifh. At this 

 junrtn-e, fir Ifaac Newton, who was a good mechanician, 

 as well as mnthematician and experiment.il philofopher, took 

 the fubjeft into his confideration, and, by his fucccfsful 

 labonrs, prevented tlie invention from faUing into oblivion. 

 His proceedings met with interruption from the occurrence 

 of the plague; but about the end of the year 1668, he be- 

 gan his experiments on fpeculum metal, and, in the year 

 1672, produced two fmall refleding telefcopes. I:i thefe, 

 the large fpecula were ground into a fpherical concave fur- 

 faccj as being the cafieft to execute ; but he was aware that 

 the p:i: ibolic curve, recommended by Gregory, would be 

 preferable, when it could be accomplifhed by mechanical 

 contrivances, which he judged to be within the reach of 

 human ingenuity. The refult of thefe 'abours was communi- 

 cated to the Royal Society of London ; and, through tlie 

 medium of their fecretary, Mr. Oldenburgh, to the in- 

 genious Huygens, who teflified liis approval of this con- 

 ftruftion in an account which was pubhfhed in the Journal 

 des S9avans for the year 1672 : and in this way, nearly the 

 whole of Europe became acquainted with the Newtonian con- 

 fi:ruftion. In the mean time, Caffegrain, a Frenchman, who 

 had varied Gregory's conflrudtion, by iubflituting a convex 

 inilead of a concave fmall fpeculum, as we have already 

 ftated, in the fame journal (des Sgavans, 1 672), contefted 

 the honour of having been the firfl improver of the original 

 Gregorian telefcope ; which claim drew from Newton fe- 

 veral objeftions to Caffegrain's conftruftion, that will indeed 

 apply equally to the Gregorian. We have, however, re- 

 cently witneffed in captain Kater an advocate for CafTe- 

 grain's telefcope, in preference to that of Gregory, (fee 

 Phil. Tranf. of London, 1813 and 1814,) principally with 

 refpeft to the brightnefs and diflinftnefs of objects refpec- 

 tively feen by them ; and his conclufion is, that much of the 

 light is difTipated by the crojftng of the rays in the focal point, 

 which is a doftrine waiting for confirmation. In Caffe- 

 grain's telefcope, the pifture of the objeft itfelf is viewed 

 by the eye ; but in Gregory's, the pifture of the image 

 reprefenting the objeft at the point of croffing is only 

 viewed ; which circumftance coiiititutes the effential dif- 

 ference in the two conflruftions : and it is very probable 

 that the light proceeding from tlie image of an objeft may 

 not be fo vivid as that proceeding from the objeft itfelf, of 

 which the image may be confidercd as a lefs enlightened 

 reprefentation. 



It is remarkable that no improvement was made on New- 

 ton's fmall telefcopes till about the year 1723, when Hadley 

 prefented to the Royal Society a reflefting telefcope of 

 Newton's conftruftion, in which the focus of the fpeculunn 

 was 10 feet ^\ inches. Though Newton's were only 

 fix inches long each, they were compared to the fix-feet 

 refraftors, fuch as were made at that time ; but what muft 

 have been the public feeUng, when Hadley produced his 

 enlarged inftrunient ! This was found at leait equal in power 

 to the famous Huygenian refraftor of 123 feet ; at leaft, its 

 power and diflinftnefs were equal, though the light was not 

 quite fo bright. 



Since Hadley's time, the reflefting telefcope has ex- 

 perienced confiderable improvements from the labours of 

 Mr. Short, Mr. Mudge, the Rev. John Edwards, Dr. 

 Herfchel (now fir William Herfchel), and others who are 

 our own contemporaries. 



But while reflefting telefcopes were undergoing their 



various improvements, and were fuperfcding the ufe of the 



long refraftors, the idea of concfting both the fphericnl and 



F f 2 prifmatic 



