THE SENSES 



777 



FIG. 281. REFRACTION BY A 

 BICONVEX LENS. 



stance is proportional to the difference of its refractive indices for the 

 extreme rays. 



Refraction by a Biconvex Lens. A straight line ACB passing 

 :nrough the centres of curvature of the two surfaces of the lens is 

 called the principal axis. A point C 

 lying on the principal axis between the 

 two centres of curvature, and possess- 

 ing the property that rays passing 

 through it do not suffer refraction, 

 is called the optical centre of the lens. 

 Any straight line, DCE, passing through 

 the optical centre is a secondary axt's. 

 Rays of light proceeding from a point 

 in the principal axis are focussed in a 

 point on that axis. When the rays pro- 

 ceed 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. Similarly, rays 

 parallel to, or 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 secondary and the principal axis must not be 

 so large as is indicated in Fig. 281. 



Formation of Image by Bi- 

 convex Lens (Fig. 282). 

 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 



. T,^ FIG. 282. FORMATION OF IMAGE BY 

 and the secondary axis BC. BICONVEX LENS. 



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 erect. 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 bya Biconcave Lens (Fig. 283). 

 Parallel rays are rendered divergent by the 

 lens; there is no real focus, but if the rays 

 are prolonged backwards they meet in the 



virtual focus F, from which they appear to come when received by the 

 eye through the lens. 



