1831.] on Vibrating Elastic Surfaces. 357 



perceive waves travelling as it were across the cloud in opposite 

 directions, they could be most distinctly traced. This is 

 exactly the appearance that would be produced by a dusty 

 atmosphere lying upon the surface of a plate and divided into 

 a number of alternate portions rapidly expanding and contracting 

 simultaneously. 



126. The spaces were very many times too small to repre- 

 sent the interval through which the air by its elasticity would 

 vibrate laterally once for two vibrations of the plate, in analogy 

 with the phenomena of liquids ; and this forms a strong ob- 

 jection to its being an effect of that kind. But it does not seem 

 impossible that the air may have vibrated in subdivisions like 

 a string or a long column of air ; and the air itself also being 

 laden with particles of lycopodium would have its motions ren- 

 dered more sluggish thereby. I have not had time to extend 

 these experiments, but it is probable that a few, well-chosen, 

 would decide at once whether these appearances of the particles 

 in the air are due to real lateral vibrations of the atmosphere, 

 or merely to the direct action of the vibrating plate upon the 

 particles. 



127. If the atmosphere vibrates laterally in the manner sup- 

 posed, the effect is probably not limited to the immediate vicinity 

 of the plate, but extends to some distance. The vertical plates 

 intersecting the surface of water and vibrating in a horizontal 

 plane (117) produced ripples proceeding directly out from them 

 five or six inches long ; whilst the waves parallel to the vibrating 

 plate were hardly sensible ; and something analogous to this 

 may take place in the atmosphere. If so, it would seem likely that 

 these vibrations occurring conjointly with those producing 

 sound, would have an important influence upon its production 

 and qualities, upon its apparent direction, and many other of 

 its phenomena. 



128. Then by analogy these views extend to the undulatory 

 theory of light, and especially to that theory as modified by 

 M. Fresnel. That philosopher, in his profound investigations 

 of the phenomena of light, especially when polarized, has con- 

 ceived it necessary to admit that the vibrations of the ether 

 take place transversely to the ray of light, or to the direction 

 of the wave causing its phenomena. " In fact we may conceive 

 direct light to be an assemblage, or rather a rapid succession, 



