529 



DIOPTRIC MECHANISMS OF THE EYEBALL 



lens at Q, will leave the lens at R in direction RS parallel to PQ. The point 

 o where the ray cuts the optical axis is known as the optical centre of the 

 lens. This optical centre in a bi-convex lens lies within the lens, its distance 

 from the two surfaces being practically as their radii. 



FIG. 257. 



Since we are neglecting the thickness of the lens the line PQRS may be 

 regarded as straight, so that we may say that the rays which pass through 

 the centre of a lens do not deviate. If a pencil of parallel rays falls upon 

 the lens, while those rays which pass through the optic centre undergo no 

 deviation, all the others on leaving the lens will be convergent towards 

 a point which is known as the principal focus of the lens (Fig. 258). Con- 



V 



FIG. 258. Diagram of the course of parallel rays through a bi-convex lens by 

 which they are converged to the principal focus, F. 



FIG. 259. 



The rays of light from A converge on passing through the lens to the 

 secondary focus, F. F and A are conjugate foci. 



versely, if a point of light be placed at the principal focus the rays of light 

 passing through it to the lens will take the reverse course and leave the 

 lens as a bundle of parallel rays. Any point of light situated between 

 infinite distance and the principal focus will have a corresponding point 

 on the other side of the lens to which its rays will converge. Such corre- 

 sponding points are known as a conjugate foci (Fig. 259). In a thin lens, 



