1870.J Light and Sound. 9 



that only the lowest sounds can be audible to that animal, but 

 that its lower limit of hearing is beneath ours.* This is consistent 

 with the doleful sounds made by that creature. Happily therefore 

 for themselves, the lowing of cattle obviously produces a totally 

 different impression upon their kindred to that which it produces 

 upon us. Moreover, movements unheard by us are probably per- 

 ceived as sound by them. If this be so at the lower limit of hearing, 

 may not similar instances occur at the higher limit ? It is perfectly 

 conceivable, nay likely, that many insects produce and hear sounds 

 far beyond our cognizance. And it would be most interesting to 

 ascertain whether the organs of hearing in a bat, for example, or a 

 grasshopper, correspond to the shrill sounds they produce. 



Returning for one moment to the illustration of the sympathetic 

 sounding of a piano, we find that when its strings are thrown into 

 vibration the motion takes time to subside. Hence we should 

 expect to find a lingering in our perception of both light and 

 sound, after the exciting cause had ceased. This is exactly what 

 occurs. The retention of light upon the retina amounts to roth 

 of a second. This retention causes a luminous point in rapid 

 motion to appear as a line ; the successive impressions blend into 

 one. We cannot perceive as distinct from each other flashes of 

 fight that succeed each other at shorter intervals than the yVth of a 

 second apart. Similarly our ear cannot distinguish between a suc- 

 cession of similar sounds that follow each other at shorter intervals 

 than the i^th of a second. Like the blending into one of the 

 colours on a spinning-top, the separate sounds link themselves 

 together and constitute a musical note. 



Radiant sound and light being both wave-motions, many laws 

 are found common to both. When light falls upon a body it is 

 either transmitted, absorbed, reflected, refracted, or inflected. The 

 same phenomena can be observed when we substitute sound for light. 

 Let us briefly examine this remarkable series of analogies. 



§ 4. Transmission of Light and Sound. 



Through transparent bodies light is transmitted freely. But 

 if a series of transparent substances, each alternately differing in 

 density, be placed together, the progress of light is obstructed, and 

 may even be altogether stopped. Thus a perfectly transparent 

 block of glass, if reduced to powder, is perfectly opaque. Air, a 

 medium of different density, now intervenes between the particles 

 of glass, and the light echoed, as it were, from particle to particle 

 is so weakened that it cannot struggle through. 



In the same way sound in its passage is enfeebled, or even 

 obliterated, if it pass through several media of alternately varying 



* Savart : ' Annales de Chimie et de Physique.' 



