Dr. Young’s Lecture 
58 
power of the lens, which is reduced by the water to a focal 
length of about 16’ tenths, (Cor. 5. Prop. IV.) is not sufficient 
to supply the place of the cornea, rendered inefficacious by the 
intervention of the water ; but the addition of another lens, of 
five inches and a half focus, restores my eye to its natural state, 
and somewhat more. I then apply the optometer, and I find 
the same inequality in the horizontal and vertical refractions as 
without the water ; and I have, in both directions, a power of 
accommodation equivalent to a focal length of four inches, as 
before. At first sight indeed, the accommodation appears to 
be somewhat less, and only able to bring the eye from the state 
fitted for parallel rays to a focus at five inches distance ; and 
this made me once imagine, that the cornea might have some 
slight effect in the natural state ; but, considering that the arti- 
ficial cornea was about a tenth of an inch before the place of 
the natural cornea, I calculated the effect of this difference, and 
found it exactly sufficient to account for the diminution of the 
range of vision. I cannot ascertain the distance of the glass 
lens from the cornea to the hundredth of an inch ; but the error 
cannot be much greater, and it may be on either side. 
After this, it is almost necessary to apologize for having 
stated the former experiments ; but, in so delicate a subject, we 
cannot have too great a variety of concurring evidence. 
IX. Having satisfied myself that the cornea is not concerned 
in the accommodation of the eye, my next object was to inquire 
if any alteration in the length of its axis could be discovered ; 
for this appeared to be the only possible alternative : and, con- 
sidering that such a change must amount to one-seventh of the 
diameter of the eye, I flattered myself with the expectation of 
submitting it to measurement. Now, if the axis of the eye 
