382 PHYSIOLOGICAL PHYSICS. [Chap. xxix. 



distance of the nodal point R= 7 '4; therefore P' 



R= 22-6470 - 7-4 = 15-2470. 



Therefore 



J_ 15-247 



O P+7-4' 



and 



15-247 



I = O 



P+7'4- 



Suppose an object 1,000 mm. (1 metre) high, seen 

 at a distance of 15-2396 metres (15239-6 mm.), what 

 is the size of the retinal image ? 



15-247 

 1>0 ' "15239-6 + 7-4 



= 1*0 mm. 



That is, at a distance of rather more than 15 

 metres, the image is a thousand times smaller than 

 the object. 



The visual angle is usually defined as the 

 angle included by the lines from the extreme points 

 of the object where they cross at the nodal point, 

 the angle x enclosed by the lines A and B of 

 Fig. 171. Helmholtz, however, has shown that the 



visual angle is properly the 

 angle enclosed by the visual 

 \ lines, which are lines from a 

 point in space which pass through 

 the centre of the image of the 

 Fig. 171. Visual Angle. pupil formed by the cornea, and 



pass to the centre of the yellow 



spot. The apparent size of objects depends upon 

 the visual angle. Thus the objects c d e all form 

 the same angle x, and thus appear to the eye to be 

 of the same size. The size of the angle depends 

 on (1) the size of the object, and (2) its distance from 



