THE VISIBLE IS ONLY ITS SHADOW. 



57 



extend the powers of those organs, we are still looking out upon 

 our surroundings in, a very rudimentary manner; we have indeed 

 to acknowledge that the human race is so much in its infancy 

 that our eyes and other organs of perception can hardly be said 

 to be yet opened. 



The two organs by which we principally gain knowledge of 

 our surroundings are those of sight and hearing, and I will now 

 demonstrate to you how narrow is the possible range within 

 which they can be used in our attempt to investigate the world 

 of appearances. 



What w^e call hearing is the apprehension of vibrations in 

 matter, mostly in the form of the air we breathe ; and when these 

 vibrations strike the ear in regular succession, beyond a certain 

 number in a second, they produce the effect of what may be 

 called a solid or continuous sound, namely, a musical note. If 

 a number of these notes are sounded together, we call it a noise. 

 Below sixteen vibrations in a second the ear can hear them as 

 separate beats but beyond that number the sound is continuous. 

 If I had no regard for your feelings I could have arranged to 

 illustrate this by loud explosions or pistol shots fired in quick 

 succession, and up to fifteen explosions in a second you would 

 have heard them separately, and the noise would have been so 

 terrific that I should no doubt have quickly lost you all as on 

 audience ; but if you could have endured the pain you would 

 have had a great surprise when the rate had reached sixteen 

 explosions in a second ; as if by magic the harsh noise would 

 suddenly have disappeared and in its place, though the explosions 

 were still going on, you would have heard a wonderful deep 

 musical sound like that given out by the longest pipe of an organ. 

 I have however arranged the experiment in a gentler fashion and 

 its demonstration will be pleasant instead of painful. 



I have here a large metal disc, which can be revolved at a 

 high speed, and 1 have had holes drilled regularly on it in con- 

 centric circles ranging from sixteen up to five hundred in the 

 different circles. We will arrange for a puff of air to be forced 

 through each hole singly as it is brought round by the revolution 

 of the disc, and when the puffs occur at a lower rate than sixteen 

 in a second you can hear them as puffs, but beyond that number 

 you will hear them as a musical sound and each of the circles 

 will give a special note according to the number of holes therein. 

 On the same disc I have also drilled in concentric circles a 

 sequence of holes, in the exact ratio necessary for combining 

 harmonies, and you can hear that from puffs of air from a 

 single nozzle can be produced the principal chords of the m.usical 

 clef. 



As already stated, the lowest musical note the ear can hear 

 is formed by sixteen vibrations in a second, the octave above 



