THE SENSE OF SIGHT 



aodal point, which latter becomes the center of curvature of the single re- 

 dacting surface. The dimensions of this "reduced" eye are as follows (see 

 : ?ijr 206) From the anterior surface of the cornea, corresponding to the 

 brincipal plane H, to the nodal point N, 5.215 mm., from the anterior focal 

 point F , to the principal plane H, i.e., the anterior focal distance/ , 15.49 

 tarn from the principal plane H to the posterior focal point F v i.e., the 

 boste'rior focal distance/", 20.713 mm.; the index of refraction is 1.3365. 

 There is thus substituted for the natural eye a single refracting surface with 

 la radius of curvature, r, of 5.125 mm. In such an eye luminous rays emanat- 

 ing from the anterior focal point are parallel to the axis after refraction in 

 the interior of the eye. Also rays parallel to the axis before refraction unit. 



at 'fly mean7of f this reduced eye the construction of the refracted ray, the 

 various calculations as to the size of the image, the size of diffusion c es, 



hJ*^*?^ an object. From A a pencil of rays falls 

 on the single refracting surface. One of the rays, the nodal ray, falling on 

 Ae surface perpendicularly, passes unrefracted through the single nodal point, 



TT FIG 207 THE FORMATION OF AN IMAGE IN THE 



FIG. 296. THE REDUCED EYE. REDUCED EYE. 



N to the posterior focal plane. The remaining rays, partially r. 

 n the figure falling on this surface under varying degrees of 



of curvature, r, are known, by the following formula: 



O :I=D+r:f"-r. 



For, as the triangles A N B and a N b are similar, we ^^ ^ 

 A B- a b=fN:N g, or a fc^^X' and therefore / p+r' 



