LIGHT. 209 



i 



bodies of different densities it is bent or made to deviate from its 

 course ; and such deviation is called refraction ; the ray is said to be 

 refracted; and, owing to its being susceptible of such refraction, is 

 held to be refrangible. The point, at which a ray enters the medium, 

 is called the point of immersion ; and that, by which it issues from 

 such medium, the point of emergence. Instead of considering the me- 

 dium I J opaque, let it be regarded as transparent. C, in this case, 

 will be the point of immersion for the incident rays that meet there ; 

 and L and F will be the points of emergence for the rays K E and A 

 C F G, respectively. If a ray of light, as K C, falls perpendicularly 

 on the surface of any medium, it continues its course through it with- 

 out experiencing any modification, and emerges in the same straight 

 line. Hence a. body at L will appear in its true direction and distance 

 to an observer at K looking directly downwards on a pool of water, I J. 

 If, on the other hand, a ray, as A C, after having passed through air, 

 falls obliquely upon the surface of the water B ; by entering a medium 

 of different density, it is deflected from its course ; and, instead of pro- 

 ceeding in the direction C H, it is refracted, at the point of immersion, 

 in the direction C F that is, towards the perpendicular K E. If, 

 again, the ray emerges at F into a medium of the same density as that 

 through which it passed in the course A C, it will proceed in a line 

 parallel to A C, or in the direction F G, or will wander from the per- 

 pendicular. The cause of this difference in the deflections produced 

 by different media is not easy of explanation. The fact alone is known 

 to us, that bodies refract light differently according to their densities 

 and nature. If the light proceeds from a rarer to a denser medium, it 

 is attracted or refracted towards the perpendicular ; if, on the contrary, 

 it passes from a denser to a rarer medium, it is refracted from the per- 

 pendicular. The ray A C proceeded from a rarer medium, the air, 

 into a denser, I J water; it was refracted in the direction C F, 

 towards the perpendicular K E. On emerging at F, circumstances 

 were reversed ; it wandered from the perpendicular M N, and in the 

 direction F G, parallel to A C, because the media, above and below I 

 J, were identical. We can now understand, why water, saline solutions, 

 glass, rock-crystal, &c., have higher refractive powers than air. They 

 are more dense. 



The nature or character of bodies greatly influences their refractive 

 powers. Newton observed this in his experiments, and has furnished 

 science with one of its proudest trophies, by his prognostic, in the then 

 infant state of chemistry, that water and the diamond would be found 

 to contain combustible ingredients. The diamond or brilliant is one of 

 the most refractive of known substances, and this is one of the sources 

 of its brilliancy. The opinion of Newton, it is hardly necessary to 

 say, has been triumphantly confirmed. 



This refraction of rays, that fall obliquely upon a medium, gives rise 

 to numerous optical illusions. The ray proceeding from F, in the bent 

 course F C A, will impinge on an eye at A ; and the object F will ap- 

 pear to be at/. The pool will consequently seem shallower. In like 

 manner, an object in the air would not be perceptible to an eye in 

 the water at F, in the direction C F ; whilst one at A would be dis- 



VOL. I. 14 



