CHAPTER IV. 



PHENOMENA OF LIGHT AND COLOUR, HEAT 

 AND SOUND. 



It is said that, after Aristotle had published a work on 

 some esoteric part of his philosophy, Alexander the Great 

 wrote to him from Asia complaining of his attempting 

 thus to communicate to people generally what had pre- 

 viously been imparted to Aristotle's select pupils only. 

 Aristotle replied to the effect that no harm could be done 

 by the publication complained of, because what he had put 

 forth in his lectures on the more abstruse parts of his 

 philosophy could be understood only by those who heard 

 him and by nobody else. The work referred to is con- 

 sidered by some to have been Aristotle's Akroasis Physike, 

 commonly called the Physics. The above story may or 

 may not be true, but it is undeniable that many parts 

 of Aristotle's works on essentially abstruse subjects are very 

 difficult to understand, and among such parts are those 

 relating to light and colour. 



The history of the development of this branch of human 

 knowledge reveals, it is true, many great achievements, but 

 it probably reveals many more disappointing failures, and 

 little of any practical importance was done until about the 

 twelfth century. Successful investigation of phenomena of 

 light and colour has been largely the result of careful 

 observation and ingenious experiments, and few, if any, 

 branches of natural science better exemplify a laborious, 

 step by step, advance to the truth. Aristotle's achieve- 

 ments, judged by the standard of knowledge in more 

 modern times, were of little value, although they must 

 have cost him much time and labour, as may be seen from 

 his account of the causes of rainbows, already given in 

 Chapter iii. 



The ancient emission or corpuscular theory of light held 

 by Empedocles, Democritus, and many other ancient philo- 

 sophers was rejected by Aristotle. He says that light is 



