4 >6 
Dr. Young's Lecture 
and very liable to inflammatory affections ; and, in order to 
make the sight so perfect as it is, it was necessary to confine that 
perfection within narrow limits. The motion of the eye has a 
range of about 55 degrees in every direction ; so that the field 
of perfect vision, in succession, is by this motion extended to 
110 degrees. 
But the whole of the retina is of such a form as to receive 
the most perfect image, on every part of its surface, that the 
state of each refracted pencil will admit ; and the varying den- 
sity of the crystalline renders that state more capable of deline- 
ating such a picture, than any other imaginable contrivance could 
have done. To illustrate this, I have constructed a diagram, 
representing the successive images of a distant object filling the 
whole extent of view, as they would be formed by the succes- 
sive refractions of the different surfaces. Taking the scale of 
my own eye, I am obliged to substitute, for a series of objects 
at any indefinitely great distance, a circle of 10 inches radius ; 
and it is most convenient to consider only those rays which pass 
through the anterior vertex of the lens ; since the actual centre 
of each pencil must be in the ray which passes through the 
centre of the pupil, and the short distance of the vertex of the 
lens from this point, will always tend to correct the unequal 
refraction of oblique rays. The first curve (Plate IV. Fig. 16.) 
is the image formed by the furthest intersection of rays refracted 
at the cornea ; the second, the image formed by the nearest in- 
tersection ; the distance between these, shows the degree of con- 
fusion in the image ; and the third curve, its brightest part. Such 
must be the form of the image which the cornea tends to deli- 
neate in an eye deprived of the crystalline lens ; nor can any 
external remedy properly correct the imperfection of lateral 
