3O2 Refrafting Power of different Mediums. [Book III. 



But if a ray AC paffcs obliquely from air into wa- 

 ter, inftead of continuing its conrle in the direct line 

 C B, it takes the direction C a, and approaches the 

 perpendicular p P, in fuch a manner that the angle of 

 refraction PCa is lefs than its angle of incidence 

 $C A. 



If the ray came in a more oblique direction, the re- 

 fraction would be ftill greater; fo that in all cafes, 

 where the mediums are the fame, the angle of refrac- 

 tion will always be found to bear a regular and con- 

 .ftant proportion to the angle of incidence; or, to fpeak 

 in technical language, the fine of incidence is to the 

 fine of refraction in a given ratio, and this ratio is dif- 

 covered by experience. Thus, when a 'ray paffcs out 

 of air into water, the ratio is as 4 to 3. 

 out of water into air, as 3 to 4. 



air into glafs, as 3 to 2 *. 



glafs into air, as 2 to 3. 



air into diamond, as 5 to 2. 



diamond into air, as 2 to 5. 



The refraction of light, we have already feen, is at- 

 tributed by Sir Ifaac Newton to the principle of at- 

 traction; and perhaps one of the moft fatisfactory 

 proofs of this theory is the known fact, that the change 

 in the direction of the ray commences, not when it 

 comes in contact with the refracting medium, but a 

 little before it reaches the furface, and the incurvation 

 augments in proportion as' it approaches this medium. 

 Indeed no principle will account for the phenomenon of 

 light pafling more eafily, that is, more directly, through 

 a denfe than through a rare medium, but that of at- 



* There are feme differences in the refra&ive powers of dif- 

 ferent glafTcs, according to the nature of the materials ; but thefe 

 are too minute to deferv notice in treating of general principles. 



traftion, 



