on Muscular Motion . 
3 
that in an achromatic object glass we obtain in a less perfect 
manner, by proportioning the radii of curvature of the different 
lenses. In the eye, the correction seems perfect, which in the 
object glass can only be an approximation, the contrary aber- 
rations of the lenses not having the same ratio ; so that if 
this aberration be perfectly corrected at any given distance 
from the centre, in every other it must be in some degree im- 
perfect. 
Pursuing the same comparison : In the achromatic object 
glass, we may conceive how much an object must appear fainter 
from the great quantity of light lost by reflection at the sur- 
faces of the different lenses, there being as many primary re- 
flections as there are surfaces ; and it would be fortunate if 
this reflected light was totally lost. Part of it is again reflected 
towards the eye by the interior surfaces of the lenses, which 
by diluting the image formed in the focus of the object glass, 
makes that image appear far less bright than it would other- 
wise have done, producing that milky appearance so often 
complained of in viewing lucid objects through this sort of 
telescope. 
In the eye the same properties that obviate this defect, 
serve also to correct the errors from the spherical figure, by a 
regular diminution of density from the centre of the crystal- 
line outward. Every appearance shews the crystalline to con- 
sist of laminae of different densities ; and if we examine the 
junction of different media, having a very small difference of 
refraction, we shall find that we may have a sensible refrac- 
tion without reflection : now if the difference between the 
contiguous media in the eye, or the laminae in the crystal- 
line, be very small, we shall have refraction without having 
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