SCT. XX. TRANSMISSION OP LIGHT 171 



bodies, brings another class of phenomena under the 

 laws of the undulatory theory. 



The ethereal medium pervading space is supposed to 

 penetrate all material substances, occupying the inter- 

 stices between their molecules; but in the interior of 

 refracting media it exists in a state of less elasticity 

 compared with ks density in vacuo ; and the more 

 refractive the medium, the less the elasticity of the 

 ether within it. Hence the waves of light are trans- 

 mitted with less velocity in such media as glass and 

 water than in the' external ether. As soon as a ray of 

 light reaches the surface of a diaphanous reflecting sub- 

 stance, for example a plate of glass, it communicates its 

 undulations to the ether next in contact with the surface, 

 which thus becomes a new center 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 E less velocity than the inci- 

 dent waves ; and the other is transmitted back into the 

 air, with a velocity equal to that with which -it' came 

 (N. 198). 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 ve- 

 locity. The particles of ether without the glass, which 

 communicate 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. Conversely, 

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

 similar to large balls striking small ones. The small 

 balls receive a motion which would cause the refracted 

 ray, and the part of the motion retained by the large 

 ones would occasion the reflected wave ; so that when 

 light passes through a plate of glass or of any other 

 medium differing in density from the air, there is a 

 reflection at both surfaces ; but this difference exists 

 between the two reflections, that one is caused by a 

 vibration in the same direction with that of the incident 

 ray, and the other by a vibration in the opposite direction. 



A single wave of air or ether would not produce the 



