THE REALITIES OF SCIENCE 61 



What are the causes of the sensations which we call 

 light? What is the objective reality of such phenom- 

 ena? Let us attempt to answer these questions by 

 considering in detail an illustration of sound, a similar 

 subjective reality. 



We are all familiar with some musical instrument and 

 know that sound is produced from a stringed instru- 

 ment by setting the string in motion. Its natural 

 motion is periodic, for it takes the same time for each 

 swing back and forth through the position of rest from 

 which it is displaced. The smaller the period of this 

 vibration, the higher is the pitch of the musical note 

 which we recognize. 



The phenomena are most easily observed it produced 

 in a slightly different manner. Let a visiting card be 

 held so as to touch the teeth of a rotating gear wheel. 

 The card is pushed away and flaps back, that is, vi- 

 brates, once for every tooth which comes in contact 

 with it. If the wheel is turning very slowly we recognize 

 the flapping of the card as a regular or rhythmic noise. As 

 the speed of the wheel increases this becomes a musical 

 note of rising pitch. The number of complete vibrations 

 which the card makes in each second is called its " vi- 

 bration frequency" and is the measure of the pitch. 



Consider now the means by which the motion of the 

 sounding body reaches the ear of the listener. The 

 intervening air consists of small discrete particles or 

 molecules. As the vibrating card is pushed out by a 

 tooth it forces ahead of it the adjacent layer of mole- 

 cules, which in turn push against those adjacent to 

 them. A city crowd, gathered around some object 

 of interest, as it surges away under the commands and 



