228 PRACTICAL PHYSIOLOGY 



CHAPTER LXVI (Advanced). 

 THE EYE AS AN OPTICAL INSTRUMENT. 



Preliminary Consideration of the Mechanism of the Eye. In 

 order to understand the refraction of the rays proceeding from external 

 objects and forming images on the retina, it is necessary, in the first 

 place, to briefly consider the nature of such an optical system as 

 constitutes the refractive apparatus of the eye. 



The simplest form of an optical system consists of two media of 

 different refractive powers separated by a spherical surface (Fig. 209). 



If dpe be such a surface, separating a less refractive medium IS from a 

 more strongly refractive medium R, n is the centre of curvature, and is 

 called the "nodal point." If p be the vertex of the curved surface, a 

 line through p and n will form the optic axis OA. Rays parallel 

 to OA proceeding from S will be conveyed to a point F 2 on the optic 

 axis. This point is called the posterior principal focus. Rays parallel 

 to OA proceeding from R will be conveyed to a point F lt the principal 

 anterior focus, p is spoken of as the principal point. These two 

 foci, the principal point and the nodal point, constitute the cardinal 

 points of such a system. 



In the actual eye the arrangement is not so simple, as there are 

 several refractive media, and three separate surfaces the anterior 

 surface of the cornea, the anterior surface of the lens, and the 

 posterior surface of the lens. The arrangement of these is, however, 

 symmetrical, and permits of the reduction to two ideal surfaces for the 

 three actually existing. This brings the number of cardinal pcints to 

 six, as each of these surfaces will possess its own nodal point and 

 principal point, though the anterior and posterior foci will be identical. 



But for practical purposes a further simplification is possible. The 

 two nodal points are not far separated, and the two principal points are 

 similarly very near, being distant only about '4 mm. from each other. 

 We therefore take a "mean" nodal point and a "mean" principal 

 point and again reduce the optical conditions to those of a simple 

 optical system, consisting of one (ideal) refractive surface. In such 

 a "reduced eye" the cardinal points are as follows : 



Principal point. 2-3448 mm. behind the anterior surface of the 

 cornea, in the aqueous humour. 



