SECT. XX. ABSORPTION OF LIGHT. 177 



the number of internal reflections." Thus Sir John Herschel, 

 by referring the absorption of light to the subdivision and mutual 

 destruction of the vibrations of ether in the interior of bodies, 

 b rings another class of phenomena under the laws of the undu- 

 latory theory. 



According to Mr. Eankin's hypothesis of Molecular Yortices* 

 the absorption of light and radiant heat consists in the trans- 

 ference of motion from the molecules to their atmospheres, and 

 conversely the emission of light and radiant heat is the transmis- 

 sion of motion from the atmospheres to the molecules. The 

 great velocity of light and heat is a natural consequence of this 

 hypothesis, according to which the vibratory masses must be ex- 

 tremely small compared with the forces exerted by them. 



The ethereal medium pervading space is supposed to penetrate 

 all material substances, occupying the interstices between their 

 molecules ; but in the interior of refracting media it exists in a 

 state of less elasticity compared with its density in vacuo ; and, 

 the more refractive the medium, the less the elasticity of the 

 ether within it. Hence the waves of light are transmitted with 

 less velocity in such media as glass and water than in the ex- 

 ternal ether. As soon as a ray of light reaches the surface of a 

 diaphanous reflecting substance, for example a plate of glass, it 

 communicates its undulations to the ether next in contact with 

 the surface, which thus becomes a new centre of motion, and two 

 hemispherical waves are propagated from each point of this 

 surface ; one of which proceeds forward into the interior of the 

 glass, with a less velocity than the incident waves ; and the 

 other is transmitted back into the air, with a velocity equal to 

 that with which it came (N. 203). Thus, when refracted, the 

 light moves with a different velocity without and within the 

 glass ; when reflected, the ray comes and goes with the same 

 velocity. The particles of ether without the glass, which com- 

 municate their motions to the particles of the dense and less 

 elastic ether within it, are analogous to small elastic balls striking 

 large ones ; for some of the motion will be communicated to the 

 large balls, and the small ones will be reflected. The first would 

 cause the refracted wave, and the last the reflected. Con- 

 versely, when the light passes from glass to air, the action is 

 similar to large balls striking small ones. The small balls 

 * See page 104. 



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