LECT. XX. ADAPTATION OF THE EYE. 371 



By looking at the object directly, vision would be distinct 

 at these different distances ; the eye, therefore, must undergo 

 some modification in order to bring about this result. 



We ought, then, to be able to find in the intimate struc- 

 ture of the eye, some peculiarities calculated to explain the 

 faculty we possess of seeing distinctly at every distance. 

 The different media of the eye have been long compared 

 to an apparatus composed of lenses terminated by regular 

 surfaces, and having all their axes on the same line repre- 

 sented by the axes of the eye itself. On this hypothesis, 

 all the luminous rays emanating from any point of an object 

 ought to be concentrated at a single point called the focus ; 

 and in order that the vision might be distinct, it would be 

 necessary that the retina should be so placed that the dif- 

 ferent foci, corresponding to the different points of an ex- 

 terior object, should be formed at its surface. But as the 

 place at which the image is formed by refraction, approaches 

 or recedes from the refracting apparatus, just as the object 

 itself recedes from or approaches it, we ought to find in the 

 structure of the eye some means of remedying this shifting of 

 the image, by which the distinctness of the impression pro- 

 duced upon the retina itself may be maintained. For this 

 purpose several hypotheses have been advanced. 



1st. It has been thought that the transparent cornea 

 might vary its curvature so as to remedy this change of 

 place of the image. But observation has proved that this 

 curvature was invariable. 



2dly. Some physiologists fancied that the crystalline lens 

 had the power of contracting, and that the curvatures of its 

 two surfaces could change, so as to keep the image con- 

 stantly upon the retina. Everything proves that this is pure 

 hypothesis. 



3dly. Setting out from this fact, that the pupil dilates 

 when the object recedes, and proportionately contracts as, 



