4 5 
on the Mechanism of the Eye. 
the external limits formed by the different parts of the face, 
when the eye is directed forwards and somewhat downwards, 
which is its most natural position ; although the internal limits 
are a little more extensive than the external ; and both are well 
calculated for enabling us to perceive the most readily, such 
objects as are the most likely to concern us. Dr. Wollaston’s 
eye has a larger field of view, both vertically and horizontally, 
but nearly in the same proportions, except that it extends further 
upwards. It is well known, that the retina advances further 
forwards towards the internal angle of the eye, than towards 
the external angle ; but upwards and downwards its extent is 
nearly equal, and is indeed every way greater than the limits of 
the field of view, even if allowance is made for the refraction 
of the cornea only. The sensible portion seems to coincide 
more nearly with the painted choroid of quadrupeds : but the 
whole extent of perfect vision is little more than 10 degrees ; or, 
more strictly speaking, the imperfection begins within a degree 
or two of the visual axis, and at the distance of 5 or 6 degrees 
becomes nearly stationary, until, at a still greater distance, vision 
is wholly extinguished. The imperfection is partly owing to 
the unavoidable aberration of oblique rays, but principally to 
the insensibility of the retina : for, if the image of the sun 
itself be received on a part of the retina remote from the axis, 
the impression will not be sufficiently strong to form a perma- 
nent spectrum, although an object of very moderate brightness 
will produce this effect when directly viewed. It would probably 
have been inconsistent with the economy of nature, to bestow a 
larger share of sensibility on the retina. The optic nerve is at 
present very large ; and the delicacy of the organ renders it, 
even at present, very susceptible of injury from slight irritation, 
