372 VISION. LECT. XX. 



the object approaches, natural philosophers have supposed 

 that the vision of distant objects was effected by means of 

 the rays traversing the less refracting marginal portion of 

 the crystalline ; whilst the act of vision at short distances 

 was performed exclusively by means of the rays passing 

 through the more refracting layers of the centre of the crys- 

 talline lens. In this manner the image would be always 

 distinctly formed at the surface of the retina. It is needless 

 to show here, that this explanation is at least very far from 

 being complete. 



4thly. Some physiologists have had recourse to a change 

 of place of the crystalline in the interior of the globe of the 

 eye, in order to explain vision at every distance. But 

 there is no evidence that such a movement of the lens is 

 effected, and it is difficult to conceive the possibility of it. 



5thly. Finally, it has been thought that the contraction of 

 the muscles of the eye, and the consequent pressure on this 

 organ, were sufficient to lengthen or shorten its axis at will ; 

 and, consequently, to change the position of the retina, and 

 incessantly restore it to a suitable position for receiving the 

 distinct image of the external object. This, also, is a pure 

 hypothesis, which has nothing to support it. 



The distinctness of vision at every distance remained, 

 then, inexplicable, and seemed to have escaped all the re- 

 searches of physiologists and philosophers, when Sturm 

 placed the question on its true ground, and clearly showed 

 why all previous attempts to explain it had failed. 



According to Chossat's measurement of the eye of an ox, 

 the anterior surface of the cornea is a segment of an ellip- 

 soid of revolution about the major axis of the ellipse, which 

 represents the horizontal section of this cornea ; and the 

 faces of the crystalline are segments of ellipsoids of revolu- 

 tion about the lesser axes of their generating ellipses; the 

 axes of these two ellipses not exactly coinciding in length. 



