2 Light and Sound. [Jan., 



essentially the same. Here, however, we have solely to deal with 

 the physical aspect of the question. In pursuit of our object it will 

 be necessary to compare the principal phenomena of light and 

 sound, and for this purpose it will be convenient to break up the 

 subject into sections. If the analogy be just, it will assuredly gather 

 strength as the comparison proceeds ; if it be false, then each section 

 cannot fail to force this fact upon the mind. In either case the 

 result ought to be profitable, if we simply seek the truth. 



§ 1. Origin of Light and Sound. 



Light and sound are both the products of vibratory motion. 

 But to evolve light the motion must be enormously swift, whilst to 

 produce sound the motion must be comparatively slow. In the 

 former case only impalpable molecules can be made to attain the 

 requisite swiftness — light is therefore a molecular motion of vibra- 

 tion. In the latter case visible masses of matter can be moved to 

 and fro with the necessary speed : sound is therefore usually the 

 product of a molar motion of vibration. Further, to continue the 

 light, or to sustain the sound, the to-and-fro motion must be per- 

 formed in equal times ; it must be isochronous. If not isochronous 

 the light will be either intermittent or varying in colour, and the 

 sound will be either a noise or musical notes of varying pitch. 



Now comes a remarkable point. Sound and even music are 

 usually produced by a disturbance very different from a vibratory 

 motion. If we hit a tuning-fork on our knee, strike the strings of 

 a piano, or pluck the strings of a harp, we produce music by rough 

 mechanical means ; so the noise of hammering, the roar of cataracts, 

 the whistling of the wind, or "the scream of a maddened beach," 

 are all sounds, that is motions of vibration, produced by a rude 

 motion of translation. Light, also, can be evolved by similar 

 agency. The rubbing of two pieces of quartz or sugar, the sparks 

 from a flint or steel, and the incandescence produced by the friction 

 of meteors against the air are familiar examples of light generated 

 by mechanical means. 



How can we account for the transition from molar to molecular 

 motion, from an impulse, such as a blow, to a regular pendulum-like 

 swing ? A well-established law of mechanics is no doubt the true 

 explanation. This law may be stated as follows : * — " That if a body 

 receive a shock and sufficient time be allowed to elapse so that the 

 initial disturbance is destroyed by friction, imperfect elasticity and 

 other causes, the final resultant motion will be vibratory and isochro- 



* This appears to be a fundamental law of the universe; namely, that an 

 original impulse of any kind finally resolves itself into periodic or lythmic motion. 

 Does not this throw light upon the periodic motion of planets as well as the 

 vibratory motion of atoms ? Possibly, in some such way, we may hereafter learn 

 to understand the musical role of nature. 



