On the Mechanl/m of the Eje, 4jt 



Dr. WoUafton's eye has a larger field of view, both vertically and horizontally, bttt nearly 

 in the fame proportions, except tliat 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 

 refradlion of the cornea only. The fenfible portion feems to coincide more nearly with the 

 painted choroid of quadrupeds : but the whole extent of perfeft vifion is little more than 

 lo degrees ; or, more ftriftly fpeaking, the imperfection begins within a degree or two of 

 the vifual axis, and at the diftance of 5 or 6 degrees becomes nearly ftationary, until, at a 

 flill greater diftance, vifion is wholly extinguiflred. The imperfeftion is partly owing to 

 the unavoidable aberration of oblique rays, but principally to the Infenfibility of the retina : 

 for, if the image of the fun itfelf be received on a part of the retina remote from the axis, 

 the imprefllon will not be fufiiciently ftrong to form a permanent fpe£lrum, although an 

 objedt of very moderate brightnefs will produce this efFedt when dlreflly viewed. It would 

 probably have been inconfiftent with the economy of nature, to beftow a larger (hare of 

 fenfibility on the retina. The optic nerve is at prefent very large ; and the delicacy of the 

 organ renders it, even at prefent, very fufceptible of injury from flight irritation, and very 

 liable to inflammatory affeftions; and, in order to make the fight fo perfeft as it is, it was 

 neceflary to confine that perfe£lion within narrow limits. The motion of the eye has a 

 range of about 55 degrees in every direftion ; fo that the field of pcrfedl vifion, in fucccf- 

 Con, is by this motion extended to 1 10 degrees. 



But the whole of the retina is of fuch a form as to receive the mod perfect image, on 

 every part of its furface, that the ftate of each refrafled pencil will admit ; and the varying 

 denfity of the cryftalline renders that ftate more capable of delineating fuch a piflure, than 

 any other imaginable contrivance could have done. To illuftrate this, I have conftrufled a 

 diagram, reprefenting the fucceffive images of a diftant objedl filling the whole extent of 

 view, as they would be formed by the fucceflive refra£lions of the different furfaces. 

 Taking the fcale of my own eye, I am obliged to fubftitute, for a feries of objeds at any 

 indefinitely great diftance, a circle of 10 inches radius ; and it is moft convenient to con- 

 fider only thofe rays which pafs through the anterior vertex of the lens; fince the adlual 

 centre of each pencil muft be in the ray which pafles through the centre of the pupil, and 

 the ftiort diftance of the vertex of the lens from this point, will always tend to correct the 

 unequal refradlon of oblique rays. The firft curve (Plate XV. Fig. 16.) is the image 

 formed by the furtheft interfe£l:ion of rays refraded at the cornea ; the fecond, the image 

 formed by the neareft interfe£lion ; the diftance between thefe, (hows the degree of con- 

 fufion in the image ; and the third curve, its brighteft part. Such muft be the form of the 

 image which the cornea tends to delineate in an eye deprived of the cryftalline lens ; nor 

 can any external remedy properly correft the imperfedlion of lateral vifion. The next three- 

 curves (how the images formed after the refra£lion at, the anterior furface of the lens, 

 diftingui(hed in the fame manner ; and the three following, the refult of all the fuccelfivc 



P p i refradions. 



