

SENSE OF SIGHT. 433 



upright ones (a and b) appearing in the cornea (a), and in 

 the anterior surface of the crystalline lens (b) (convex mir- 

 rors) ; and the one upside down (c), on the posterior surface 

 of the crystalline lens (concave mirror). If the person ex- 

 perimented upon be made to look fixedly at objects placed 

 at different distances, the only change in the three reflections 

 which we have mentioned will be found to take place in that 

 furnished by the anterior surface of the 

 crystalline lens (the reflection b). We 

 conclude from this, that in the phenom- 

 enon of accommodation the anterior sur- 

 face of the crystalline lens alone undergoes 

 a change ; while, by measuring the image 

 in question, we find (according to the 

 laws of convex mirrors) that, in looking 

 at an object at a distance, the convexity 

 of the crystalline lens is diminished (since 

 we find that the image increases in size) ; 

 while if, on the other hand, the object is g^ e ?J ^ irr e 8 1 j 8 * (Pur ~ 

 near, the convexity is increased (the image 

 being then reduced in size). 



Therefore, accommodation takes place by the modification 

 of the crystalline lens. We shall examine the other means 

 by which the form of this lens may be changed, when we 

 come to the subject of those accessory membranes, especially 

 the choroid and the iris (the ciliary muscle) which are in- 

 tended to assist and to modify the working of the essential 

 parts of the eye. 



D. Imperfections in the /System of Ocular Dioptrics. Con- 

 sidered as a physical organ, the eye is far from being perfect : 

 the various imperfections belonging to similar physical organs 

 :tre found in it, and are known under the name either of 

 spherical aberration or chromatic aberration. 



The essential part of the organ of the eye being a lens, it 

 so happens that this lens, even when most perfect, does not 

 unite exactly at the same point all the rays which, proceed- 

 ing from the same luminous source, fall upon the edges or 

 upon the centre of the crystalline lens. The focus of the lens 

 is not, therefore, single in its kind, and it is this which gives 

 rise to the aberration of sphericity. We shall see that the 



* a, Upright image reflected by the cornea. 6, Upright image reflected by 

 the anterior surface of the lens. *c, Image upsidedown reflected by the pos- 

 terior surface of the lens. 



28 



