PHYSIOLOGICAL OPTICS 



799 



same plane as the incident ray and the perpendicular drawn to the 

 surface, separating the two media. The second law is that the ratio 

 which the line of the incident ray bears to the line of the angle of 

 refraction, is constant for the same two media but different for differ- 

 ent media. 1 



Refractive media may be bounded by: 



(a) Two plane surfaces which are parallel to one another. A ray impinging 

 upon a plate at a right angle, traverses this medium without suffering a change in 



FIG. 409. DIAGRAM IL- 

 LUSTRATING REFRACTION BY 

 A PLATE- LIKE BODY. 



B c 



FIG. 410. DIAGRAM ILLUS- 

 TRATING REFRACTION BY PRISMS. 



its direction. Any other ray DB meeting this surface at an angle, is bent toward 

 the perpendicular AB on entering, but away from it on leaving the medium. The 

 emergent ray EF is parallel to the incident ray (Fig. 409). 



(6) Two plane surfaces which incline toward one another. At the point of 

 intersection of these two surfaces is the summit or apex A. Their inclination 

 constitutes the refractive angle and their right line EC the base. The medium so 

 outlined is a prism (Fig. 410). A ray of light LE impinging upon one of its lateral 

 surfaces AB, is deflected toward the normal P at E, because it passes into a more 

 highly refractive medium. It here forms the angle of incidence LEE and the angle 



3 456 



FIG. 411. DIFFERENT FORMS OF CONVEX AND CONCAVE LENSES. 



of refraction IEF. When meeting with the other surface AC it is again refracted, 

 the angle of refraction HFK being greater than the angle of incidence EFI ; because 

 it passes from a more highly refractive medium into one of less power. Thus, the 

 ray is deflected from its course in the direction of the base of the prism. In this 

 case, the image of L is produced at S, in the prolongation of the emerging ray. 



(c) Two surfaces one of which is either curved or plane. The refractive medium 

 is thus arranged in the form of a lens which in accordance with its shape may be 

 spherical, cylindrical, elliptical or parabolic. In optics spherical lenses are most 

 commonly employed and they may be made of crown glass or flint glass. The 

 former is free from lead and is therefore less refractive than the latter which contains 

 lead. By combining spherical surfaces either with plane or curved surfaces, six 



1 Stated by Snell in 1620, but enunciated by Descartes. 



