THE SENSES 



785 



tions of the same lines towards the retina. This angle is 

 called the visual angle, and evi- 

 dently 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 comparative nearness 

 of the earth's satellite more than 

 makes up for its relatively small 



FIG* 290. THE REDUCED EYE. 

 The dimensions of the retinal image thfi s - n s le spherical refracting 



- , . ., i i j i surface, 2 '2 mm. behind the an- 



of an object are easily calculated when te rior surface of the cornea; N, ! 



the size of the object and its distance the nodal point, 5 mm. behind S ; 



are known. For let AB in Fig. ,9, ..* ff+f~ < T ' 



represent One diameter Of an Object, cornea and lens are put in in dotted 



A'B' the image of this diameter, and lines in the position which they 



let AB', BA', be Straight lines passing *ipy in the normal eye. 



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. 



FIG. 291. FIGURE TO SHOW HOW THE VISUAL ANGLE AND SIZE OF RETINAL 

 IMAGE VARIES vvirii THE DISTANCE OF AN OBJECT OF GIVEN SIZE. 



For the distant position of AB the visual angle is , for the near position (dotted 



lines) /3. 



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



and d its distance from B', we have = -=-. Now, d may 



a 



approximately be taken as 15 mm. Suppose, then, that the size of 

 the moon's image on the retina is required. Here D=- 238,000 miles, 

 and AB (the diameter of the moon) = 2,1 60 miles. Thus we pet 



50 



