ox THE MECHANISM OF THE EYE. 



595 



■lent, even if no other were found. I do 

 not however doubt, that in those who have a 

 large pupil, and great power of changing 

 the focus, the iiherration may 1)6 very fre- 

 quently observable. In Dr. Wullaston's eye, 

 the diversity of appearance is im[)erceptible ; 

 but Mr, Konifi: described the intersections 

 exactly as they appear to me, altliough he 

 had received no hint of what I had observed. 

 The lateral refraction is the most easily ascer- 

 tained, by substituting for the slits a taper- 

 ing piece of card, so as to cover all the cen- 

 tral parts of the pupil, and thus determining 

 the nearest crossing of tlie sliadovvs trans- 

 mitted through the marginal parts only. 

 When the furthest intersection was at S8, I 

 could bring it to 22 parts with two narrow 

 slits; butwiih the tapered card only to 29. 

 From these data we may determine pretty 

 nearl}', into wluit form ihc lens must be 

 changed, supposing both the surfaces to un- 

 dergo propor(ional alterations of curvature, 

 and taking for granted the dimensions al- 

 ready laid down : for, from the lateral aber- 

 ration thus given, we may find the subtan- 

 gents at about one tenth of an inch from the 

 axis; and the radius of curvature, at each ver- 

 tex, is already determined to be about 21 

 and 15 hundredths of an inch. Hence, the 

 anterior surface must be a portion of a hy- 

 perboloid, of which the greater axis is al out 

 50; and the posterior surface will be nearly 

 parabolical. In this manner, the change 

 will be eflected, without any diminution of 

 the transverse diameter of the lens. The 

 elongation of its axis will not exceed the 

 fiftieth of an inch ; and, on the supposition 

 with whieii we set out, the protrusion will 

 be chiefly at the posterior vertex. The form 

 of the lens, thus changed, will be nea:iy that 

 of Platen. Fig. 90; the relaxed state be- 



ing nearly as represented in Fig. 89. Should, 

 however, the rigidity of the internal and 

 more refractive parts, or any other consider- 

 ations, render it convenient to .suppose the 

 anterior surface more changed, it would still 

 have room, without interfering with the 

 uvea ; or it might even force the uvea a little 

 forwards, without any visible alteration of 

 ■ the external appearance of the eye. 



Why, and in what cases, such an imperfec- 

 tion must exist in the lateral refraction, is 

 easily understood, from the marginal attach- 

 ment of the lens to its capsule. For,if the cur- 

 vature at the axis be increased in any consi- 

 derable degree, it cannot be continued far to- 

 wards the margin, without lessening the dia- 

 meter of the lens, and tearing the ramifications 

 which enter it from the ciliary processes. Nor 

 does there appear to be any other reason 

 for the very observable contraction of the 

 pupil, which always accompanies the effort 

 to view near objects, than that by this 

 means the lateral rays are excluded, and 

 the indistinctness is prevented, which would 

 have arisen from the insuflSciency of their re- 

 fraction. 



From this investigation of the change of 

 the figure of the lens, it appears that the ac- 

 tion, which I formerly attributed to the exter- 

 nal coats, cannot afford an explanation of 

 the phenomenon. The necessary effect of 

 such an action would be, to produce a fi2;ine 

 approaching to that of an oblate spheroid ; 

 and, to say nothing of the inconvenience at- 

 tending a diminution of the diameter of the 

 lens, tlie lateral refraction would be much more 

 increiised than the central; nor would the 

 slight change of density, at an equal distance 

 from the axis, be at all equivalent to the in- 

 crease of curvature : we must therefore sup- 

 pose some different mode of action in the 



