750 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



The curvature of the refracting surfaces of the eye is determined by an 

 instrument known as an ophthalmometer, which measures the size of the 

 reflected image of a known object in the various curved surfaces. The 

 radius of curvature of the surface is determined by the following formula : 



r 

 B :b=A : - j or r = , in which B = the size of the object, 6 = the size of 



2 jD 



the image, A = distance between the object and the reflecting surface, and 

 r = the radius of the reflecting surface. The distances between the various 

 surfaces of the eye are measured on frozen sections of the organ, or can be 

 determined upon the living eye by optical methods too complicated to be here 

 described. It should be borne in mind that the above values of the so-called 

 "optical constants" of the eye are subject to considerable individual variation, 

 and that the statements of authors concerning them are not always consistent. 

 The refracting surfaces of the eye may be regarded as still further sim- 

 plified, and a so-called " reduced eye " constructed which is very useful for 

 purposes of optical calculation. This reduced eye, which for optical purposes 

 is the equivalent of the actual eye, is regarded as consisting of a single refract- 

 ing medium having an index of 1.33, a radius of curvature of 5.017 milli- 

 meters, its principal point 2.148 millimeters behind the anterior surface of the 

 cornea, and its nodal point 0.04 millimeter in front of the posterior surface 

 of the lens. 1 The principal foci of the reduced eye are respectively 12.918 

 millimeters in front of and 22.231 millimeters behind the anterior surface of 

 the cornea. Its optical power is equal to 50.8 dioptries. 2 The position of this 

 imaginary refracting surface is indicated by the dotted line in figure 216. The 



FIG. 216. Diagram of the formation of a retinal image (after Foster). 



nodal point, n, in this 'construction may be regarded as the crossing-point of all 

 the principal rays which enter the eye, and, as these rays are unchanged in their 

 direction by refraction, it is evident that the image of the point whence they 

 proceed will be formed at the point where they strike the retina. Hence to 

 determine the size and position of the retinal image of any external object 

 e. g. the arrow in Figure 216 it is only necessary to draw lines from various 



1 Strictly speaking, there are in this imaginary refracting apparatus which is regarded as 

 equivalent to the actual eye two principal and two nodal points, each pair about 0.4 millimeter 

 apart. The distance is so small that the two points may, for all ordinary constructions, be 

 regarded as coincident. 



2 The optical power of a lens is the reciprocal of its focal length. The dioptry or unit of 

 optical power is the power of a lens with a focal length of 1 meter. 



