784 A MANUAL OF PHYSIOLOGY 



arrived at, by considering a single homogeneous medium, of 

 definite refractive index, and bounded in front by a spherical 

 surface of definite curvature, to replace the transparent 

 solids and liquids of the eye. The principal focus being 

 supposed to lie on the retina, the position of the nodal point 

 (i.e., the point through which rays pass without refraction) 

 of such a * reduced ' or ' schematic ' or ' simplified ' eye, 

 and other constants, are shown in the following table. The 

 single refracting surface would be situated behind the cornea 

 and in front of the lens, at a rather smaller distance from 

 the anterior surface of the latter than from the anterior 

 surface of the former. The nodal point would be less than 

 half a millimetre in front of the posterior surface of the lens 

 (Fig. 290, p. 785). The refractive index of the single trans- 

 parent medium would be a little greater than that of 

 water. 



Reduced Eye 



Radius of curvature of the single refracting surface - 5-1 mm. 

 Index of refraction of the single refracting medium - -1*35* ,, 

 Antero posterior diameter of reduced eye (distance of 



principal focus from the single refracting surface) - 20.0 

 Distance of the single refracting surface behind the 



anterior surface of the cornea 2*2 



Distance of the nodal point of the reduced eye from 



its anterior surface 5*0 ,, 



Distance of the nodal point from the principal focus 



(retina)- - 15*0 



Knowing the position of the centre of curvature of the 

 single ideal refracting surface, i.e., the nodal point of the 

 reduced eye, all that is necessary in order to determine the 

 position of the image of an object on the retina is to draw 

 straight lines from its circumference through the nodal 

 point. Each of these lines cuts the refracting surface at 

 right angles, and therefore passes through without any 

 deviation. The retinal image is accordingly inverted, and 

 its size is proportional to the solid angle contained between 

 the lines drawn from the boundary of the object to the 

 nodal point, or the equal angle contained by the prolonga- 



* Or about the same as that of the aqueous humour. 



