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XXL An Attempt to explain a Difficulty in the Theory of Vijion , 
depending on the different Refrangibility of Light . By the 
Rev . Nevil Mafkelyne, D. D . fA R . & AJironomer 
Royal . 
Read June 1 8, 1 789. 
fTP^HE ideas of fight are fo fir iking and beautiful, that we 
JL are apt to confider them as perfectly diftindt. The 
celebrated Euler, taking this for granted, has fuppofed, in 
the Memoirs of the Royal Academy of Sciences at Berlin 
for 1747, that the feveral humors of the human eye were 
contrived in fuch a manner as to prevent the latitude of 
focus arifing from the different refrangibility of light, and 
confiders this as a new reafon for admiring the ftrudture of the 
eye; for that a fingle tranfparent medium, of a proper figure, 
would have been fufficient to reprefen t images of outward 
objedts in an imperfect manner ; but, to make the organ of 
fight abfolutely complete, it was neceffary it fhould be com- 
pofed of feveral tranfparent mediums, properly figured, and 
fitted together agreeable to the rules of the fublimeil geome- 
try, in order to obviate the effedt of the different refrangibility 
of light in diflurbing the diflinclnefs of the image ; and hence 
he concludes, that it is poflible to difpofe four refracting fur- 
faces in fuch a manner as to bring all forts of rays to one 
focus, at whatever diftance the object be placed. He then 
affumes a certain hypothefis of refraction of the differently 
refran- 
