802 



THE SENSE OF SIGHT 



it would if the rays entering it had been parallel. This is the function of the 

 cornea. It tends to gather the slightly divergent rays and to render them available 

 for refraction by the lens. This discussion shows that if an object, even a very 

 large one, is placed at a sufficient distance from a biconvex lens, a small real and 

 inverted image of it is formed just outside the principal focus F. The greater the 



Fig. 



417. — Diagram Illustrating the Refraction of Extremely Divergent Rays by 

 A Biconvex Lens 



distance, the smaller this image. This principle is illustrated by our eye as well as 

 by the ordinary photographic camera. Quite similarly, one small object placed 

 upright just outside the principal focal point F oi a, biconvex lens, forms a large 

 inverted image at a considerable distance in front of the lens. This principle is 

 illustrated by the projection lantern. 



Fig. 418. — Dla.gram Illustrating Formation of an Image by a Biconvex Lens. 



In constructing the image of an object AB as formed by a biconvex lens, it must 

 be remembered that one ray AD emitted by luminous point A, always traverses the 

 nodal point of the lens N unrefracted and that a second ray AE enters the lens paral- 

 lel to its principal axis (Fig. 418). The ray AE is then refracted through the focal 

 point F. The focal point of A lies at the point of intersection of these two hnes. 



Fig. 419. — Diagram Illustrating the Refraction by a Biconcave Lens. 



If this construction is now extended to a luminous point B upon the lower end of the 

 object AB, it will be seen that this one is brought to a focus above. Consequently, 

 the image of object AB is inverted. In those cases in which the object is placed 

 between the biconvex lens and its principal focus, only virtual erect images are 

 formed This principle is made use of in the construction of microscopes and 

 magnifying 



