106 THE REDUCED EYE. 



which represents what the normal eye would be, if its 

 refracting surfaces were spherical, their centres in the same 

 axis, and its transparent media homogeneous. 



The lensless or reduced Eye. If it were not necessary that 

 the eye should be capable of being adjusted for the distinct 

 vision of objects of different distances, the lens would not 

 be required ; for an eye which consists of but one medium, 

 and has but one refracting surface, answers the dioptrical 

 purposes of the real eye in every respect, excepting that it 

 cannot be " accommodated." A schema of this kind is 

 called a " reduced eye." In such an eye, if the radius of 

 curvature of the cornea is 5'I2, and the index of refraction 

 about i "35, the conditions approach pretty closely to those 

 of the normal eye. In the reduced eye the straight line 

 which passes through the centre of the cornea and the 

 centre of its sphere of curvature is the axis. Rays which 

 are in the same line with a radius of the refracting surface 

 are not refracted : such rays are called principal rays. As 

 they all pass through the centre of the sphere of curvature 

 of the cornea, that centre is called the " crossing point." 

 In the normal eye this point lies immediately in front of 

 the posterior surface of the lens. Any number of rays 

 reaching the cornea from a luminous point (object) at 

 sufficient distance in the axis, are so refracted at the surface 

 that they converge to a point (image) on the other side. 

 Rays which emanate from a luminous point in a plane 

 including the first, which is vertical to the axis (object- 

 plane) converge to a point in the same vertical plane with 

 the image of the first point (image-plane). It thus happens 

 that (as regards flat surfaces of small extent which face 

 the cornea) every point of the object-plane is focussed in 

 the image-plane, forming there an inverted image. The 

 further the object-plane is from the refracting surface, the 

 nearer must be the image-plane. The point to which the 

 almost parallel rays which emanate from any very distant 

 point in ,the axis converge, is called the principal 



