VISION 



971 



two centres of curvature, and possessing the property that rays passing 

 through it do not surfer refraction, is called the optical centre of the 

 lens. Any straight line, DCE, passing through the optical centre, is a 

 secondary axis. Rays of light proceeding from a point in the principal 

 axis are focussed in a point on that axis. When the rays proceed from 

 an infinitely distant point in the principal axis i.e., when they are 

 parallel to it they are focussed in F, the principal focus. Smilarly 

 rays parallel to, cr 

 proceeding from, a 

 point in a secondary 

 axis are focussed in 

 a point on that axis ; 

 but if the focus is to 

 be sharp, the angle 

 between the secon- 

 dary and the princi- 

 pal axis must not be 

 so large as is indi- 

 cated in Fig. 392. 



Formation of Im- 

 age by Biconvex Lens 

 (Fig- 393)- Let AB 

 be the object; then 

 if AHD be the path 

 of a ray from A parallel to the principal axis, the image of A will be the 

 intersection of the straight line DF and the secondary axis passing 

 through A. Similarly, the image of B will be the intersection of GF 

 and the secondary axis BC. Where AB is farther from the lens than 

 the principal focus, the image ab is real and inverted. This is the case 

 with the image of an external object formed on the retina. When the 

 object is nearer than the principal focus, the image is virtual and direct. 



Fig. 391. Refraction and Dispersion by a Prism. 



392. Refraction 

 Biconvex Lens. 



Fig. 



393. Formation of Image 

 Biconvex Lens. 



by 



The image formed by the objective of a microscope when the object is in 

 focus is real and inverted ; the ocular forms a virtual erect image of this 

 real image. 



Refraction by a Biconcave Lens (Fig. 394). Parallel rays are rendered 

 divergent by the lens; there is no real focus; but if the rays are pro- 

 longed backwards they meet in the virtual focus F, from which they 

 appear to come when received by the eye through the lens. 



Formation of Image by Biconcave Lens (Fig. 395). Let AB be the 

 object. Let AUDI be the path of a ray from any point A of the object 



