582 



ON THE MECHANISxM OF THE EYE. 



cornea only. The sensible portion seems to 

 coincide more nearly with the painted cho- 

 roid of quadrupeds : but the whole extent of 

 perfect vision is little more than 10 degrees; 

 or, more strictly speaking, the iniperfection 

 begins within a degree or two of the visual 

 axis, and at the distance of 5 or 6 degrees 

 becomes nearly stationar}', until, at a still 

 greater distance, vision is wholly extin- 

 guished. 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 re- 

 mote from the axis, the inipression will not 

 be sufficieiitly strong to form a permanent 

 spectrum, although an object of very mode- 

 rate brightness will produce this effect when 

 directly viewed. It has been said, that a 

 faint light, like the tail of a comet, is more 

 observable by a lateral than by a direct view. 

 Supposing the fact certain, the reason pro- 

 bably is, that general masses of light and 

 shade -are more distinguishable when the 

 parts are somewhat confused, than when the 

 whole is rendered perfectly distinct; thus I 

 have often obiierved the pattern of a paper os 

 floor cloth to run in certain lines, when I 

 viewed it without my glass ; but these lines 

 vanished as soon as the focus was rendered 

 perfect. It would probably have been in- 

 consistent with the economy of nature, to 

 bestow a larger share of sensibility on the re- 

 tina. 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, and very liable to in- 

 flammatory afiections; and, in order to make 

 the sight so perfect as it is, it was necessary 

 to confine that perfection within narrow li- 

 mits. The motion of the eye has a range of 



ab()uf55 degrees in every direction : so that 

 t'le field of perfe^^t vision, in succession, is by 

 this motion extended to 1 10 d.egrees. 



But the whole of the retina is of such a 

 form as to receive the most perfect image, on 

 ever3' part of its surface, that tlie state of 

 each refracted pencil will admit ; and the va- 

 rying density of the crystalline renders that 

 state more capable of delineatiirg such a pic- 

 ture, than any other imaginable contrivance 

 could have done. To illustrate this, I have 

 constructed a diagram, representing the suc- 

 cessive images of a distant object filling the 

 whole extent of view, as they would be 

 formed by the successive 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 dis- 

 tance, a circle of 10 inches radius; and it is 

 most convenient to consider only those mys 

 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 refrac- 

 tion of oblique rays. The first curve (Plate 

 10. Fig. 80) is the image formed by the 

 furthest intersection of rays refracted at the 

 cornea; the second, the image formed by 

 the nearest intersection ; the distance, be- 

 tween these, shows the degree of confusion 

 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 vision. 

 The next three curves show the images 

 formed after the refraction at the anterior 

 surface of the lens, distinguished in the same 



