LECT. XX. 



ADAPTATION OF THE EYE. 



Moreover, the axes of these three generating ellipses of the 

 surfaces of the refracting media of the eye, coincide neither 

 with the axis of the eye nor with each other. 



It follows, then, that in place of comparing the optical 

 apparatus of the eye to a system of spherical lenses, whose 

 axes were blended, we ought, according to Sturm, to con- 

 sider the organ as " composed of several refracting media, 

 separated by surfaces which are neither exactly spherical, 

 nor even of revolution or symmetrical about a common 

 axis." 



Sturm, studying the problem in all its generalities, has 

 shown that, with a like composition of the eye, the fasci- 

 culus of luminous rays, transmitted to the cornea by a 

 point placed on the prolongation of the axis of the eye, 

 could not be so refracted that all the rays could converge 

 towards a single focus ; but the following is what hap- 

 pens: 



Fig. 33. 



Diagram Explanatory of Sturm's Hypothesis of Vision at different Distances. 



Let the circle ABA'B' represent the aperture of the pupil, 

 and ox the axis of the eye : then suppose that a fasciculus 

 of rays, parallel to the axis, falls upon the cornea. 



It will have there two planes, AOA' and BOB', perpendi- 

 cular to each other, so that all the luminous rays contained 

 in the plane A o A', will converge towards the axis in one 

 focus only, F, and all the rays contained in the plane B o B', 

 will be concentrated towards the axis in one point /. Let 

 us call the distance F/", the focal interval. 



