304 PHILOSOPHICAL Tl!AN3ACTIONS. [aNNO 1740. 



The first of these defects only, was known to the writers on dioptrics, before 

 Sir Isaac Newton ; for which reason, as he informs us. Opt. Lect. 1 , 2, " they 

 imagined, that optical instruments might be brought to any degree of perfec- 

 tion, provided they were able to communicate to the glasses, in grinding, what 

 geometrical figure they pleased ; to which purpose various mechanical con- 

 trivances were thought of, whereby glasses might be ground into hyperbolical, 

 or even parabolical, figures ; yet nobody succeeded in the exact description of 

 such figures ; and had their success been answerable to their wishes, yet their 

 labour would have been lost (continues this incomparable mathematician) ; for 

 the perfection of telescopes is limited, not so much for want of glasses truly 

 figured, according to the prescriptions of optic authors, (which all men have 

 hitherto imagined) as because that light itself is an heterogeneous mixture of 

 dift^erently refrangible rays ; so that were a glass so exactly figured as to collect 

 any one sort of rays into one point, it could not collect those also into the 

 same point, which having the same incidence upon the same medium, are apt 

 to sufi^er a difl^erent refraction." Phil. Trans. N° 80. And again, — " Diversa 

 diversorum radiorum refrangibilitas impedimento est, quo minus optica, per 

 ' figuras, vel sphaericas, vel alias, perfici possint ; nisi corrigi possint errores 

 illinc oriundi, labor omnis in caeteris corrigendis imperite collocabitur," Prin- 

 cipia, &c. Scholium ad finem Libri Primi. 



Now, for this principal and last-mentioned defect, no one has proposed any 

 remedy; apprehending perhaps the difficulty of attaining such to be insuperable ; 

 inasmuch as the great author of this discovery himself had not shown any me- 

 thod to correct those errors which arise from this inequality of refraction ; but 

 rather discouraged any such attempts, by declaring, " that on this account he 

 laid aside his glass-works," (Phil. Trans, N° 80) " and looked upon the im- 

 provement of telescopes, of given lengths, by refraction, as desperate." Optics, 

 ad Edit. p. 91. 



However, as it has been proved by incontestible experiments, that this dis- 

 sipation of the rays of light, from whatever cause it proceeds, in passing out of 

 one medium into another, is not accidental and irregular ; but that every sort of 

 homogeneal rays, whether more or less refrangible, considered apart, are re- 

 fracted according to some constant uniform and certain law ; and as the re- 

 moval of so great an impediment as this, of unequal refraction in the rays of 

 light, is of great importance to the science of dioptrics, and absolutely neces- 

 sary to its further advancement ; we have thought it worthy of a careful exami- 

 nation, whether, in some cases at least, it might not be possible for contrary 

 refractions so to correct each other's inequalities, as to make their difference 

 regular ; and if this could be conveniently eflTected, Sir Isaac Newton has 

 acknowledged, " there would be no further difficulty." Phil. Trans. N* 88. 



