LAWS OF REFRACTION. 6 



will give the sine of the latter. In accordance with these laws, a 

 ray of light passing from one medium to another perpendicularly, 

 undergoes no refraction ; and of several rays at different angles, 

 those nearer the perpendicular are refracted less than those more 

 inclined to the refracting surface. When a pencil of rays, however, 

 impinges on the surface of a denser medium (as when rays passing 

 through Air fall upon Water or Glass), some of the incident rays are 

 reflected from that surface, instead of entering it and undergoing 

 refraction ; and the proportion of these rays increases with the in- 

 crease of their obliquity. Hence there is a loss of light in every case 

 in which pencils of rays are marie to pass through Lenses or Prisms : 

 and this diminution in the brightness of the image formed by 

 refraction will bear a proportion, on the one hand, to the number of 

 surfaces through which the rays have had to pass ;and on the other, 

 to the degree of obliquity of the incident rays, and to the diffe- 

 rence of the refractive powers of the two media. Hence in the 

 passage of a pencil of rays out of Glass into Air, and then from Air 

 into Glass again, the loss of light is much greater than it is when 

 some medium of higher refractive power than air is interposed 

 between the two glass surfaces ; and advantage is taken of this 

 principle in the construction of Achromatic combinations for the 

 Microscope, the component lenses of each pair or triplet (§ 13) 

 being cemented together by Canada Balsam ; whilst it is also 

 applied in another mode in the ' immersion lenses ' recently brought 

 into use on the Continent (§ 14). On the other hand, advantage 

 is taken of the partial reflexion of rays passing from air to glass 

 at an oblique angle to the surface of the latter, in the construction 

 of the ingenious (non-stereoscopic) Binocular of Messrs. Powell and 

 Lealand (§ 62). 



2. On the other hand, when a ray w o emerges from a dense 

 medium into a rare one, instead of following the straight course, it 

 is bent from the perpendicular, according to the same ratio ; and 

 to find the course of the emergent ray, the sine of the angle of 

 incidence must he multiplied by the ' index of refraction,' which 

 will give the sine of the angle of refraction. Now when an 

 emergent ray falls very obliquely upon the surface of the denser 

 medium, the refraction which it would sustain in passing forth 

 into the rarer medium, tending as it does to deflect it still farther 

 from the perpendicular, becomes so great that the ray cannot 

 pass out at all, and is reflected back from the plane which separates 

 the two media, into the one from which it was emerging. This 

 internal reflection will take place, whenever the product of the 

 sine of the angle of incidence, multiplied by the index of refrac- 

 tion, exceeds the sine of 90°, which is the radius of the circle ; 

 and therefore the 'limiting angle,' beyond which an oblique ray 

 suffers internal reflexion, varies for different substances in propor- 

 tion to their respective indices of refraction. Thus, the index of 

 refraction of Water being 1'336, no ray can pass out of it into a 



n 2 



