1905-1906.] Address by tJie President. 321 



transparent substances possess a power of refraction greater 

 than that of air, or, as it is theoretically called, a vacuum. 

 Thus, if the power of refraction of air be taken as unity 

 (1), the power of other media will be greater than unity. 

 The measure of this power is called the refractive index of the 

 medium. Thus if air be taken as 1, water will be 1 "3 3, oil 

 of cedar-wood 1'52, and crown-glass 1*5 2. Again, the re- 

 fractive index of any particular medium will differ according 

 to the ray of light employed. It will be least for the red 

 and greatest for the violet, and the difference between these 

 powers of refraction is termed the power of dispersion of the 

 medium. 



When a ray of light enters a denser medium from a rarer 

 one, it is bent or refracted towards the normal ; and con- 

 versely, when a ray of light enters a rarer medium from a 

 denser, it is refracted away from the normal. The normal is 

 a line drawn perpendicular to the plane surface of the medium, 

 but if the surface be spherical then the normal is a radius of 

 the curvature. The ray falling upon the medium is called 

 the incident ray, and the angle it makes with the normal is 

 called the angle of incidence ; while the ray in the medium is 

 called the refracted ray, and the angle which the refracted ray 

 makes with the normal is called the angle of refraction. The 

 angle of incidence and the angle of refraction are measured as 

 between the ray and the normal, and not between the ray and 

 the surface of the medium. The diagram on Plate XXV., 

 Fig. 1, will make the matter clear. 



Let F Gr be the plane surface of a transparent medium 

 upon which a ray of light A C is incident ; draw the normal 

 B D perpendicular to F G at the point C. Let the incident 

 ray make with the normal the angle B C A of 42°. Then if 

 the medium be water, the ray A C, instead of proceeding in a 

 straight line to E, will be refracted to W, making with the 

 normal the angle W C D of 30° ; while if the medium be oil 

 of cedar-wood or crown-glass, the ray A C will be refracted 

 to 0, making with the normal the angle C D of only 26°. 

 It is thus evident that all the light which is contained in the 

 angle of 42° in air is compressed when it enters water into 

 an angle of 30°, and when it enters oil into an angle of 26°. 

 The practical iinportance of this, in the case of the microscope. 



