590 



0N THE MECHANISM OF THE EYE. 



the anterior vertex of tlie lens, may be con- 

 sidered as delineating the image ; and, since 

 the divergence of these rays, with respect to 

 each other, is but little affected by the refrac- 

 tion of the lens, they may still be said to di- 

 verge from the centre of the pupil ; and the 

 image of a given object on the retina must 

 be very considerably enlarged, by the remo- 

 val of the retina to a greater distance from 

 the pupil and the lens. To ascertain the real 

 magnitude of the image, with accuracy, is 

 not so easy as at first sight appears ; but, be- 

 sides the experiment last related, which 

 might be employed as an argument to this 

 purpose, there are two other methods of es- 

 timating it. The first is too hazardous to be 

 of much use; but, with proper precaution?, 

 it may be attempted. I fix my eye on a brass 

 circle placed in the rays of the sun, and, af- 

 ter some time, remove it to the cancellated 

 micrometer ; then, changing the focus of my 

 eye, while the micrometer remains at a 

 given distance, I endeavour to discover whe- 

 ther there is any difference in the apparent 

 magnitude of the spectrum on the scale ; but 

 J can discern none. I have not insisted on 

 the attempt ; especially as I have not been 

 able to make the spectrum distinct enough 

 without inconvenience ; and no light is suf- 

 ficiently strong to cause a permanent impres- 

 sion on any part of the retina remote from 

 the visual axis. 1 therefore had recourse to 

 another experiment. I placed two candles so 

 as exactly to answer to the extent of the ter- 

 mination of the optic iierve, and, marking 

 accurately the point to which my eye was di- 

 rected, I made the utmost change in its fo- 

 cal length; expecting that, if there were any 

 elongation of the axis, the external candle 

 would appear to recede outwards upon the 

 1 



visible space. (Plate 9- V\g. 79.) But this 

 did not happen : the apparent place of the 

 obscure part was precisely the same as be- 

 fore. I will not undertake to say, that I could 

 have observed a very minute difference either 

 way : but I am persuaded, that I should have 

 discovered an alteration of less than a tenth 

 part of the whole. 



It may be inquired, if no change in the 

 magnitude of the image is to be expected 

 on any other supposition ; aud it will ap- 

 pear to be possible, that the changes of cur- 

 vature may be so adapted, that the magni- 

 tude of the confused image may remain per- 

 fectly constant. Indeed, to calculate froin 

 the dimensions which we have hitherto used, 

 it would be expected that the image should 

 be diminished about one fortieth, by the ut- 

 most increase of the convexity of the lens. 

 But the whole depends on the situation of 

 the refracting surfaces, and the respective in- 

 crease of their curvature, which, on account 

 of the variable density of the lens, can ' 

 scarcely be estimated with sufficient accuracy. 

 Had the pupil been placed before the cornea, 

 the magnitude of the image must, on any 

 sujiposition, have been very variable : at pre- 

 sent, this inconvenience is avoided by the 

 situation of the pupil; so that we have here 

 an additional instance of the perfection of 

 this admirable organ. 



P'lOin the experiments related, it appears 

 to be highly improbable that any material 

 change in the length of the axis actually 

 takes place : and it is almost impossible to 

 conceive by what power such a change could 

 be effected. The straight muscles, with the 

 adipose substance lying under them, would 

 certainly, when acting independently of the 

 socket, tend to flatten the eye : for, since 



