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A TEXTBOOK OF PHYSIOLOGY 



system. The second focal point is the point to which parallel rays 

 are gathered after passing through the system. 



The principal point is situated on the principal axis at the point 

 where the vertex of the spherical surface and the plane perpendicular 

 to the axis meet. Rays which pass through the first principal point 

 after refraction pass through the second principal point. 



The nodal point corresponds to the centre of curvature of the lens. 

 It is the point where a ray, falling perpendicularly on the surface, 

 and therefore passing through without refraction, cuts the axis. 

 Rays which pass through the first nodal point after refraction seem 

 to emerge from the second nodal point in a direction parallel to that 

 of the incident ray. 



FIG. 341. DIAGRAMS TO -ILLUSTRATE REFRACTION. 



In the simplest form of optical system, where there are only two 

 media separated from each other by a spherical surface (d, p, e, Fig. 341 ), 

 n, the centre of curvature of the surface, is known as the nodal 

 point. A line drawn from a through the vertex of the spherical 

 surface (p) gives the optic axis (OA). Rays parallel to OA in the 

 less refractive medium (S) will be brought to a point (F 2 ) on the optic 

 axis in the more refractive medium ( R) the posterior principal focus. 

 Rays parallel to OA, proceeding from R, will be brought to a point 

 (FJ on the optic axis the anterior principal focus. The principal 

 point is at p. 



In the eye, there are several surfaces between the various media, 

 but since the refractive indices of some are approximately the same 



