904 



A MANUAL OF PHYSIOLOGY 



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 prolongations of the same lines towards the retina. 

 This angle is called the visual angle, and evidently varies directly 

 as the size of the object, and inversely as its distance. Thus 

 the visual angle under which the moon is seen is much larger 

 than that under which we view any of the fixed stars, because 



the compara- 

 tive nearness of 

 the earth's 

 satellite more 

 than makes up 

 for its relatively 

 small size. 



The d i m e n - 

 sions of the reti- 

 nal image of an 



FIG. 386. FIGURE TO SHOW HOW THE VISUAL ANGLE object are easily 

 AND SIZE OF RETINAL IMAGE VARIES WITH THE Dis- calculated when 



TANCE OF AN OBJECT OF GlVEN SlZE. tllC size Of the 



For the distant position of AB the visual angle is a, for object and its 

 the near position (dotted lines) /3. distanceare 



known. For let 



AB in Fig. 386 represent one diameter of an object, A'B' the 

 image of this diameter, and let AB', BA', be straight lines passing 

 through the nodal point. Then AB and A'B' may be considered 

 as parallel lines, and the triangles of which they form the bases, 

 and the nodal point the common apex, as similar triangles. 

 Accordingly, if D is the distance of the nodal point from A, and 



* Or a little more than that of the aqueous humour. 



