OPTICS 261 



individual layers have various indices of refraction, the index 

 increasing as we proceed to the centre. Because of the order of 

 the layers, the actual total index of refraction is somewhat larger 

 than the index of the central layer. 



The radii of curvature are determined from the size of the 

 reflected image of a known object which is formed, by reflection, at 

 the surface. To measure the size of these images accurately the 

 ophthalmometer invented by Helmholtz is used. 



It has been found by calculation that the dioptric effect of 

 the eye can also be produced by a simple system, in which 

 the lens is not present and is replaced by vitreous humor, 

 and in which the reduction of refraction, due to omission of 

 the lens, is corrected by giving the only remaining refractive 

 surface (the surface of the cornea) a stronger curvature and 

 a different position. The system is therefore reduced to a 

 single spherical surface placed between two media (for the 

 aqueous and vitreous humor have the same index of refrac- 

 tion). The radius of this surface is 5.017 mm; the distance 

 of the centre of curvature (nodal point) from the anterior 

 surface of the cornea in the real (not reduced) eye is 7. 16 

 mm. The simplified system is called the reduced or sche- 

 matic eye, and by its aid we can construct the refracted ray 

 of light as indicated in Fig. 27. In Fig. 28 //is the position 

 of the surface of separation of the reduced eye. 



Strictly speaking the system of the eye has two nodal points 

 (A'j and A',, Fig. 28, which lie 6.96 and 7.37 mm behind the vertex 

 of the cornea), but these lie so closely together that they may be 

 regarded as one. The two nodal points have the following char- 

 acteristic. A ray which, previous to refraction, passes in the direc- 

 tion of the first nodal point, passes, after refraction, through the 

 second nodal point and parallel to its original course. Correspond- 

 ing to the two nodal points there are also two spherical surfaces. 

 The points where the optical axis cuts these two surfaces are called 

 the chief points (h^ and // 2 , Fig. 28). The first chief point lies 

 1.94 mm and the second 2.36 mm behind the anterior surface of 

 the cornea. Planes erected perpendicular to the optic axis at the 

 chief points are called chief planes. The chief planes must be 

 regarded as conjugated planes of such a nature that an object which, 

 previous to refraction, is supposed to be located in the first chit f 

 plane, must have, after refraction, an image oT the same size in the 

 second plane. 



