[ 291 ] 
fays he, it is evident, that the different refrangibility 
of the rays would make no alteration, either in the 
place of the image, or in its magnitude, if it were 
poffible to determine the radii of the four furfaces fo 
as *° hav , e » £~t) +» t+t— t) = N $~t) + 
“b n l )• And this. Sir, I fhall readily 
grant. But when the furfaces are thus proportioned, 
the fum of the refradions will be = o ; that is to fay* 
the emergent rays will be parallel to the incident 
For if n f) + m (f—j +r— f-) = N lf—f) +' 
then n—N (~—i) -f- m — M (f 
t +£ — t) = O. Alfo if n — N : 7n — M : : n — i : m j 
then n I (*— *) f) =0 . 0 ’ 
otherwife* Q-f)+m (*-*+*- = 0 , 
which reduces the denominator of the fradion ex- 
preffing the focal didance to f. Whence the focal 
didance will be = a ; or, in other words, the image 
will be the object itfelf. And as, in this cafe, there 
will be no refradion, it will be eafy to conceive how 
there fhould be no aberration. 
And now, Sir, I think I have demonftrated, that 
hdr* Eulei s theoiem is intirely founded upon a new 
law of refradion of his own ; but that, according to 
the laws difcover’d by experiment, the aberration 
arifing from the different refrangibility of light at the 
objed-glafs cannot be correded by any number of 
refradions whatfoever. I am, 
S I R, 
London, March i r, Your mod obedient humble fervant 
1 75 2 - 
John DolloncJ, 
III 
O O 2 
