594 



©* THE MECHANISJt OF THE EYE. 



and this is as great a difference as was observ- 

 ed in Benjamin Clerk's eye. It is also a 

 fact of no small importance, that Sir Henry 

 Englefield was much astonished, as well as 

 'the other observers, at the accuracy with 

 which the man's eye was adjusted to the 

 'same distance, in the repeated trials that 

 were made with itf. This circumstance 

 alone makes it highly probable, that its 

 perfect vision was confined within very nar- 

 row limits. 



Hitherto I have endeavoured to show the 

 inconveniences attending other suppositions, 

 and to remove the objections to the (-pinion 

 of an internal change of the figure of the 

 lens. I shall now state two experiments, 

 which, in the first place, come very near to a 

 mathematical demonstration of the exist- 

 ence of such a change, and, in the second, 

 explain in great measure its origin, and the 

 manner in which it is effected. 



I have already described the appearances 

 of the imperfect image of a minute point at 

 different distances from the eye, in a state 

 ■of relaxation. For the present purpose, I 

 Hvill only repeat, that if the point is beyond 

 the furthest focal distance of the eye, it 

 assumes that appearance which is generally 

 described by the name of a star, the central 

 part being considerably the brightest. (Plate 

 12. Fig. 92. n. 36. .39-) But, when the focal 

 <listance of the eye is shortened, the imperfect 

 image is of course enlarged ; and, besides 

 this' necessary consequence, the light is also 

 very differently distributed; the central part 

 becomes faint, and the margin strongly illu- 

 minated, so as to have almost the appear- 

 ance of an oval ring. (N. 41.) If I ap- 

 ply the slider of the optometer, the shadows 

 • Phil. Trans. 1795. 8. 



of the slits, while the eye is relaxed, are 

 perfectly straight, dividing the oval either 

 way into parallel segments: (N, 4'2, 44.) 

 but, when the accommodation takes place, 

 they immediately become curved, and the 

 more so the further they arc from the centre 

 of the image, to which their concavity is 

 directed. (N. 43, 45.) If the point be 

 bn)ught much within the focal distance, the 

 change of the eye will increase the illumina- 

 tion of the centre, at the expense of the mar- 

 gin. 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 curva- 

 ture than the marginal parts. If the refrac- 

 tion of the lens remained the same, it is ab- 

 solutely impossible that any change of the 

 distance of the retina should produce a cur- 

 vature in those shadows, which, in the re- 

 laxed 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 con- 

 firmed by inspection of the optometer. When [ 

 look through four narrow slits, withoutexer- 

 tion, the lines always appear to meet in one 

 point: but when I make the intersection ap- 

 proach me, thetwo outer lines meet considera- 

 bly beyond the inner ones, and the two lines 

 of the same side cross each other at a still 

 greater distance. (Plate 1 1. Fig. 88.) 



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 argu- 



