26 ADAPTING POWER OF THE EYE. 



the majority of them are situated at the centre, and will be capable of re- 

 presenting an image of the object regarded. Hence it results in Sturm's 

 opinion that whether the object be approximated closer to or removed fur- 

 ther from the eye, without any change in the eye itself, perfect vision of 

 it will be obtained, the retina being still within the limits of the intersec- 

 tion of the rays. If, however, the position of the object before the eye 

 be so much altered that none of its rays can cross each other at the retina 

 the image of it will be indistinct. 



Professor J. D. Forbes * has lately expressed an opinion that the ad- 

 justing power of the eye is due to the variable density of the crystalline 

 lens. It is usually considered that the great difference in density between 

 the central and peripheral portions of the lens is intended for the purpose 

 of correcting the effect of spherical aberration of the rays of light, but 

 Professor Forbes remarks that there is no need for the existence of such 

 correction ; for, as already mentioned, it has been shewn by M. Chossat's 

 measurements,! that the lens does not represent the segment of a sphere, 

 but of an ellipsoid, in which the surfaces have a curve of no aberration, 

 and will consequently require no variation in density of the refracting 

 medium. This Professor Forbes regards as a proof that the variable 

 density of the lens is intended for some other purpose than to correct 

 aberration : and this purpose he conceives to be the focal adjustment of 

 the eye. In order to render the lens available for the production of this 

 object, he believes that in regarding a near object the four recti muscles 

 of the eye are simultaneously and voluntarily set in action, whereby the 

 eye is drawn back into the socket, and that the pressure thus resulting 

 upon the humours of the eye is propagated to the lens, which, owing to 

 the inconsiderable density and elasticity of its peripheral parts, is altered 

 from its flattened ellipsoidal form to a somewhat more spheroidal mass, 

 and one possessed of greater density than before. In this manner its con- 

 verging powers will be increased, and distinct vision of a new object thus 

 be effected. Against this ingenious view, however, must be repeated the 

 several objections, which have been already stated at page 22 ; namely, 

 the absence of any firm resisting medium at the posterior part of the 

 socket, which is chiefly occupied by fat : no appearance, during the vision 

 of near objects, of any retraction of the globe of the eye, which probably 

 must ensue before the action of the recti muscles could exert such com- 

 pression on the lens as to produce the effect supposed by Professor 

 Forbes; and lastly, the objection urged by Volkmann against the proba- 

 bility of any share in the adjusting property of the eye being taken by 

 the recti muscles, namely, that we do not appear to possess the power of 

 voluntarily exciting all the recti muscles to simultaneous action. 



From the consideration of these several opinions offered in explanation 



* Transact, of the Royal Society of Edinburgh, 1845, part i. t Ann. de Chimie, i. c. 



