TELESCOPE. 



He alfo firft proved, that in a plano-convex lens, parallel 

 rays are made to converge to a point which is iliftant from 

 the lens jult the diameter of the fpherc of convexity ; and 

 that, if both fides of the lens are equally convex, this point 

 will be at the centre of the circle of convexity. It remained 

 however for Cavalheri to dilcover and to prove, in cafes 

 where the radii of curvature of the two fides of a double- 

 convex lens are unequal, that as the fum of both the diame- 

 ters is t« one of them, fo is the other to the diltance of the 

 focus: and it may be proper to notice here, that the fame 

 rules are applicable to concave lenfes, except that the focus 

 is at the contrary fide of the glafs. 



It is reinaikable, however, that Dcicartcs, tlie pupil of 

 Kepler, makes no mention of his tutor's improvements, in the 

 art of conllrufling a telefcope, having been carried into exe- 

 cution for feveral years after Gahlco's was brought into ufe. 

 It was not till the year 1630, that Scheincr delcribes, in his 

 " Rofa Urfina," the plan of fubftituting a convex inilead of 

 a concave eye-glafs, as fuggelled by Kepler, to be ufed for 

 aftronomical purpofcs, where the inverfion of the objeft is a 

 matter of no importance, but where the increafed field of 

 view is of material confequence. The iame mechanician 

 foon after adds a fecond convex glafs to his eye-tube, by 

 means of which the objefts become ereft, which addition was 

 no improvement to the vifion, but rather a detriment ; and 

 after him, Rheita gave an ereft pofition to objefts, by uiing 

 three fimilar lenfes in the eye-tube inilead of two, whicli 

 greatly improved the vifion, without other detriment than 

 the lofs of a little light : and becaufe Rheita's telefcope 

 was adapted for viewing objeAs on the earth, as well as in 

 the heavens, it was diflinguiflied by the name of the ter- 

 rejlrial telefcope, by way of diftinftion from Scheiner's 

 aflronomical one. In both thefe telefcopes, as well as in 

 Gahleo's with a concave eye-glafs, the power is eftimated 

 from the focus of the objeft-glafs divided by the focus of 

 tlie eye-glafs, as will be feen hereafter. 



The iludy of dioptrics now became general, and feveral 

 improvements were offered by different individuals in die 

 conftruftion of the refrafting telefcope ; but among the real 

 improvers muft be placed the very ingenious Huygens, who, 

 being well acquainted with the aberration of tlie rays of 

 light arifing from the fpherical figure of the glaffes, con- 

 trived a better arrangement of the eye-glafles than had before 

 been devifed. It was however very foon found, that the 

 power of a telefcope of any of the preceding conftruftions, 

 could not be increafed by fhortening the focus of the eye- 

 glafs alone, beyond certain limits, without introducing great 

 indiftinftnefs, arifing from the fpherical aberrations ; and 

 that the beft mode of gaining power, without diminution of 

 light and diftinftnefs, is an increafe of the focal length, with- 

 out much increafe of aperture of the objeft-glafi ; and a 

 little experience (hewed, that it is neceffary to increafe this 

 length in the duplicate ratio of the propofed increafe of 

 power : i. e. in order to magnify fwlce as much, the focus 

 of the objeft.glafs muft be made /our times as long as that of 

 another telefcope that has the fame light and dittinftnefs ; 

 and for any other power in a fimilar proportion. The con- 

 fequence of this difcovery was, that different makers began 

 to vie with, each other, with refpeft fimply to the length of 

 their telefcopes : among thefe may be mentioned Euftachio 

 Divini at Rome ; Campani at Bologna ; fir Paul Neille, 

 Mr. Reive, and Mr. Cox, in England; and in Fiance, 

 Borelli and Auzout. The laft-mentioned mechanician fuc- 

 ceeded in grinding an objea-glafs of the aftonifhing length of 

 600 feet ; and it is faid, that Hartfocker made them even 

 longer than this. 



It will here occur to the reader, that tubes of this enor- 

 10 



mous length, if pradicable, could not be manageable by an 

 obferver ; and hence we find, that tlicfe very long objeft- 

 glaffes were fixed on the top of long poles, or to growing 

 trees, and fo contrived as to be capable of adjuilment for the 

 axis of vifion when turned to different altitudes, agreeably to 

 the required pofition of the remote eye-glafs. 



But while the length of the telefcope was thus inconve- 

 niently increafed, and the trouble of making good obferv- 

 ations therewith proportionably augmented, it became a 

 queftion to determine in what proportion the aperture might 

 be enlarged with the increafe of focal length of the objeft- 

 glafs. Auzout wrote a paper, and delivered it to the Royal 

 Society in the year 1665, in which he affirmed, that the 

 diameter of the objedl -glafs ought always to be in a fub- 

 duplicate ratio of its focal length, or nearly fo ; and accord- 

 ingly drew up a table of apertures fuitable tor all focal lengths, 

 from 4 inches to 400 feet : upon which Dr. Hooke very 

 properly remarked, that the fame glafs may have its aper- 

 ture advantageoufly enlarged or diminifned, according to the 

 quantity of light proceeding from the objeft viewed. 



While powerful telefcopes were thus obliged to be un- 

 manageably long, and obtained the name of flfWa/ telefcopes, 

 from thecircumilance of their having no tubes to be inclofed 

 in, the immortal Newton had his penetrating mind occupied 

 with meditated improvements on the figure and airangement 

 of lenfes, and proceeding, as he always did, on rational prin- 

 ciples, difcovered, from the elongated and coloured fpec- 

 trum formed by rays of light palfing through a triangular 

 prifm, and from experiments calculated to inveftigate the 

 caufe of fucli an oblong form and coloured appearance, that 

 light is not homogeneous, and that different rays are dif- 

 ferently refrangible, when tranfmitted through the fame 

 medium. This grand difcovery prefented difficulties 

 Handing in the way of tlie improvement of the refradling or 

 dioptric telefcope, apparently much greater than tliofe which 

 had previoufly been difcovered, as arifing only out of the 

 fpherical figure of the glaffes ; and all hope of fuccefs in 

 making (hort telefcopes of great power, and yet with fuf- 

 ficient light and diftinftnefs, but without an admixture of 

 coloured rays, was given up. 



Yet to a mind like Newton's, it natur.ally occurred, that 

 what could not be praftically effetled by refradlion, might 

 probably be accompliihed by refleftion of the rays of light 

 into a focus, where, as there would be no feparation of the 

 colorific rays by a refrafting medium, there would be ne 

 colour nor elongation of the focal point, arifing from any 

 other aberration, than w'hat might be caufed by the figure of 

 the reflefting furface ; he therefore abandoned his propofed 

 plan of grinding lenfes after the figure of fonie of the conic 

 feftions, (for which fir Chriftopher Wren contrived a 

 machine,) to avoid the efFefts of fpherical aberration in 

 dioptric telefcopes, and turned his mind to the improvement 

 of catoptric or rather cata-dioptric telefcopes, which had 

 been previoufly propofed to Defcartes by Merfenne, and 

 aftually conilrufted by James Gregory of Aberdeen. 

 The compofition for the bell metal for refleftion, and the 

 mode of grinding an(J polilhing, as propofed and praftifed 

 by Newton, we have already detailed under our article 

 Speculum ; but as reflefling telefcopes have been con- 

 ftruftcd differently, we will here introduce a fhort account 

 of the refpeftive differences, before we refumethe remaining 

 narrative of the improvenj- nts in dioptric telefcopes. The 

 firft conftruftion of the refiefling telefcope was the Grego- 

 rian, and moil of the portable refleftors continue to be of 

 this conftrudtion at the prefent day : its large fpeculum is 

 concave, perforated at the centre, and placed at the interior 

 end of the large tube ; and the fmall rellettoris alfo concave, 



placed 



