C 1*7 1 
** bolical, Figures ; yet nobody fucceeded in the 
“ exadt Defcription of fuch Figures; and had their 
‘‘ Succefs been anfwerable to their Wifhes, yet their 
Labour would have been loft (continues this 
“ incomparable Mathematician) ; for the Perfedlion 
of Telefcopes is limited, not fo much for want of 
** Glafles truly figured, according to the Prefcriptions 
of Optic Authors, (which all Men have hitherto 
imagined) as becaufe that Light itfelf is an hetero* 
geneous Mixture of differently refrangible Rays ; 
“ fo that were a Glafs fo exadlly figured as to colleft 
any one fort of Rays into one Point, it could not 
colled thofe alfo into the fame Point, which 
having the fame Incidence upon the fame Medium, 
are apt to fuffer a different Refradion” (Thil. 
Tranf. N°. 8o.). And again, “ Diverfa.diver- 
forum Radiorum Refrangibilitas Impedimento eft, 
quo minus Optica, per Figuras, vel fphaericas, vel 
alias, perfici pofTint ; nifi corrigi poffint Errores 
illinc oriundi, Labor omnis in cacteris corrigendis 
“ imperite collocabitur’* (^rhwipiaj &c. Scholium 
eJ finem Libri 7rmi). 
Now, for this principal and laft-mentioned Defed, 
no one, that we know of, has propofed any Remedy; 
apprehending, perhaps, the Difficulty of attaining fuch 
to be infuperable; inafmuch as the great Author oL 
this Difcovery, himfelf, had not ffiewed us any Me- 
thod whereby to corred thofe. Errors which arife 
from this Inequality of Refradion; but rather dlf- 
couraged any fuch Attempts, by declaring, “ that 
‘‘ on this Account he laid afidc his Glafs-works, ” 
C\Phil. Tranf- 8o.) and looked upon the 
“ Improvement of Telefcopes, of given Lengths, 
“ by 
