150 Experimental Philosophy. [Lecture 11. 



By this figure you will understand that the 

 angle of refraction PCa is not so large as the 

 angle of incidence pCA, but bears a certain pro- 

 portion to it ; and this proportion or ratio varies 

 with respect to different mediums. Thus, when 

 a ray passes from air into water, the angle of 

 incidence is to that of refraction in the ratio of 

 about four to three ; from air into glass nearly as 

 three to two; from air into diamond nearly as 

 five to two; and the contrary proportion holds 

 in passing back again ; as when light passes from 

 water into air, the ratio is as three to four, &c. 

 From all this you will clearly understand, that 

 the more obliquely a ray falls, the greater is the 

 refraction. It is also necessary that you should 

 remember, that light is refracted or drawn towards 

 the perpendicular, (as in fig. 48), when it passes 

 out of a rare into a denser medium ; and it is re- 

 fracted from the perpendicular, or in a more ob- 

 lique direction, when it passes from a dense me- 

 dium into one which is rare ; and the denser the 

 medium, the greater is the refraction : thus the 

 diamond is found to refract most powerfully. 



This principle will explain several of the com- 

 mon phsenomena of nature. Mr. Walker ob- 

 serves, that " many a school-boy has lost his 

 life by supposing the bottom of a clear river to 

 be within his depth, as (when he stands on the 

 bank) the bottom will appear one-fourth nearer 

 the surface than it really -is. w In this case, the 



