on the Mechanism of the Eye. 6g 
modation takes place, they immediately become curved, and the 
more so the further they are from the centre of the image, to 
which their concavity is directed. (Fig. 43, 45.) If the point 
be brought much within the focal distance, the change of the 
eye will increase the illumination of the centre, at the expense 
of the margin. The same appearances are equally observable, 
when the effect of the cornea is removed by immersion in water; 
and the only imaginable way of accounting for the diversity, is 
to suppose the central parts of the lens to acquire a greater 
degree of curvature than the marginal parts. If the refraction 
of the lens remained the same, it is absolutely impossible that 
any change of the distance of the retina should produce a cur- 
vature in those shadows, which, in the relaxed state of the eye, 
are found to be in all parts straight ; and, that neither the form 
nor the relative situation of the cornea is concerned, appears 
from the application of water already mentioned. 
The truth of this explanation is fully confirmed by the opto- 
meter. When I look through four narrow slits, without exer- 
tion, the lines always appear to meet in one point : but, when I 
make the intersection approach me, the two outer lines meet 
considerably beyond the inner ones, and the two lines of the 
same side cross each other at a still greater distance. ( Plate V. 
Fig. 24.) 
The experiment will not succeed with every eye ; nor can it 
be expected that such an imperfection should be universal : but 
one case is sufficient to establish the argument, even if no other 
were found. I do not however doubt, that in those who have a 
large pupil, the aberration may be very frequently observable. 
In Dr. Wollaston’s eye, the diversity of appearance is imper- 
ceptible; but Mr. Konig described the intersections exactly as 
