PART II. 

 THE MICROSCOPE. 



FUNDAMENTAL PRINCIPLES. 



1 8. The Visual Angle. We form an opinion of the 

 magnitude of an object from the size of the image which 

 it produces on the retina. The lateral rays from any 

 object [intersect behind the crystalline lens. The size of 

 the retinal image depends on the size of the visual angle 

 that is, the angle formed by the intersection of the rays 



FIG. 4. Visual angle. 



from the extreme points of the object. All objects which 

 have the same visual angle appear of the same size. Thus 

 the objects ABC (Fig. 4), although actually of different 

 sizes, have when placed at proper relative distances from 

 the eye the same apparent size ; because the visual angle 

 is the same for all, and the magnitude of the image on 

 the retina (N) consequently the same. 



