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PHYSIOLOGY 



surface bounding the lens ; the line joining them is the optical axis of 

 the lens. The surface at Q is parallel to the surface at R, so that a 

 ray of light, such as p Q falling on the lens at Q, will leave the lens at 

 R in direction RS parallel to PQ. The point o where the ray cuts the 

 p 



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. 



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



FIG. 256. Diagram of the course of parallel rays through a biconvex lens 

 by which they are converged to the principal focus, F. 



FIG. 257. The rays of light from A converge on passing through the Iciis 

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



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. 256). Conversely, if a point of light be placed at the 



