CHAP, in.] SIGHT. 1147 



authorities, however, maintain that it does cut the retina at the 

 fovea centralis. 



705. The eye, therefore, even with the simplifications which 

 we have introduced, presents a much more complex optical system 

 than the one described above. It has, however, been shewn 

 mathematically that a complex optical system consisting of 

 several surfaces and media centred on one optical axis may be 

 treated as if it were a more simple system consisting of two 

 surfaces only. In such a simplified system each of the two (ideal) 

 surfaces has its own nodal point and its own principal foci, 

 anterior and posterior; moreover, the two points where the two 

 surfaces cut the optic axis are called principal points (and vertical 

 planes drawn through those points principal planes), first or ante- 

 rior, and second or posterior. Hence the cardinal points of such a 

 simplified complex system are six in number, namely, the anterior 

 and posterior principal foci, the anterior and posterior principal 

 points, and the anterior and posterior nodal points. (When such a 

 system is, by removal of surfaces and media, converted into the 

 still more simple system of one surface separating two media, the 

 two nodal points become coincident in one point, namely, the centre 

 of the sphere, and the two principal points become coincident in one 

 point, namely, the point at which the optic axis cuts the surface.) 



In order to effect such a simplification of a complex optical 

 system, it is requisite to know : (1) The refractive index of each 

 medium. (2) The radius of curvature of each surface. (3) The 

 distance along the optic axis between the first surface on which 

 the rays fall and the succeeding surfaces. These can be and have 

 been determined for the human eye, and the following table gives 

 the several values usually adopted with some recent corrections, 

 the latter being placed in brackets. 



Refractive index of aqueous or vitreous humour. .. 1 '3 376 (1*3 365) 



Mean refractive index of lens 1*4545 (1*4371) 



Radius of curvature of cornea 8 (7*829) mm. 



of anterior surface of lens ... 10 

 of posterior ... 6 

 Distance from anterior surface of cornea to ante- 

 rior surface of lens 4 (3*6) 



Thickness of lens 4(3*6) 



By means of these measurements the optical system of the eye 

 may be simplified into an optical system of two surfaces. In this 

 'schematic, or diagrammatic, eye of Listing,' as it is generally called, 

 the two (ideal) surfaces, and the principal points where these cut the 

 optic axis (Fig. 136, p l , p 2 , the two surfaces being indicated by 

 dotted lines), lie close together in the front part of the aqueous 

 humour, and the nodal points, n 1 , n 2 , lie, also close together, in the 

 back part of the lens. 



Further, the two principal surfaces lie so close together that, 



