258 HUMAN PHYSIOLOGY 



light, namely, for those rays which are approximately perpendicular 

 to the surface. 



3. In refraction at a spherical surface the following equation 

 expresses the distance of the luminous and image point from the 

 surface : 



^-f -? = .vV7_ ;/ '), 

 CL (i r 



1 2 



in which , is the index of refraction of the first and ;/ a that of 

 the second medium ; r is the radius of the spherical surface ; a l 



FIG. 25. 



the distance of the luminous point ; a^ that of the image point. 

 In this formula r is positive when the convexity of the surface is 

 towards the side of the luminous point, negative when it is con- 

 cave with respect to the luminons point. a l is positive when the 

 rays entering are divergent, that is, come from a real objector lumi- 

 nous point ; negative when the rays are convergent, that is, pass 

 to a virtual object point, a^ is positive for a real, negative for a 

 virtual, image point. In Fig. 26, in which O is the luminous point 

 and C the centre of curvature, all the values are positive. By 

 means of the formula the position of the image for a given posi- 

 tion of the luminous point can be found. The formula also teaches 

 that an object or luminous point placed in the image point B has 

 its image in the position of the previous luminous point O. Two 

 points, of which the one as image point has the other for its object 

 point, are called conjugated points. 



The direction of the image point from a given luminous point is 

 found by drawing a straight line from the luminous point through 

 the centre of curvature C. This straight line is called the chief or 

 directing ray, and the centre of curvature is called the crossing 



