198 TKANSMISSION OF LIGHT. [SECT. xx. 



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 un- 

 dulations to the ether next in contact with the surface, which 

 thus becomes a new centre of motion, and two hemispheri- 

 cal 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. 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 velocity. 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 glas^ 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 vibra- 

 tion 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 sen- 

 sation of sound or light. In order to excite vision, the vi- 

 brations of the molecules of ether must be regular, periodi- 

 cal, and very often repeated : and, as the ear continues to 

 be agitated for a short time after the impulse by which alone 

 a sound becomes continuous, so also the fibres of the retina, 

 according to M. d'Arcet, continue to vibrate for about the 



