280 PHYSIOLOGY CHAP. 



same path as those passing from M to M. The two points P and 

 /, that is, the object and the image, between which these reciprocal 

 relations exist, are termed conjugate foci. 



The distance of the conjugate focus / from the point S on the 

 refractive surface through which the optical axis passes, depends 

 on the distance of the luminous point P. When the distance PS 

 increases, the distance SI decreases, and vice versa', in other 

 words, the conjugate foci are displaced in the same direction. 



(c) Focal Points and Focal Planes. When rays of light passing 

 from M to M', or from M' to M, and separated by a spherical 

 surface, come from a point at infinite distance lying on the 

 principal axis, i.e. when they are parallel to each other and to the 

 principal optical axis, they converge after refraction at a point on 

 the axis known as the principal focus. This again is distinguished 

 as anterior or posterior, according as the parallel rays pass from 

 M to M' t or from M to M. 



In Fig. 119 the parallel rays represented by RA, J^^pass from 



FIG. 119. Diagram to show the principal focal points and focal planes. 



M to M', and the posterior focal point lies at F' ; the parallel rays 

 passing from M to M are represented by the dotted lines R'A and 

 RB, and the anterior focal point lies at F. The distances 

 between the two foci F'F and the refracting surface S are the 

 focal distances, FS the anterior, F'S the posterior focal distance. 

 The planes perpendicular to the optic axes, which pass through 

 the foci, are the anterior and posterior focal planes. 



(d) Construction of an Image from a given Object. Let M be 

 the first and M' the second medium (Fig. 120), AB the spherical 

 surface of separation, C its centre of curvature, FF' the anterior 

 and posterior focal points on the optical axis of the system, and 

 the object. To determine the image of in M ', draw the directive 

 line 00 which is not refracted, and then the line OH parallel to 

 the optical axis, which passes after refraction through the posterior 

 focal point F' ; point / at which the two rays intersect coincides 

 with the image of 0. So, too, the image T corresponds in M' to 

 the object 0' in M, and the whole object 00' corresponds to the 

 real inverted image //'. It is obvious from the geometrical con- 



