Sect. VII.] Optics. 59 



of power in the propagation of sound, both with 

 respect to intensity and tone, has been known 

 since the year 1786; about which time Dr. Priest- 

 ley, and professor Perolle, of Turin, instituted a 

 set of experiments on this subject, in which they 

 substantially agreed. Since that time professor 

 Jacquin, of Vienna, at the desire of Dr. Chladiii, 

 undertook a new course of experiments, witli a 

 view to the investigation of this subject. The re- 

 sults of these experiments are so different, and 

 even contradictory, ^vhen compared with the for- 

 mer, that it is difficult to say on wliich side the 

 truth lies, 



SECTION VIL 



OPTICS. ' ■ 



In this science great improvements have taken 

 place in modern times. In 1704 Sir Isaac Nc\v- 

 ton first published his grand work on Optics ; and 

 although many of his most interesting discoveries 

 were made and announced towards the close of the 

 seventeenth century, yet the collection and pub- 

 lication of them, in a systematic form, was reserved 

 to be one of the distinguishing honours of the 

 eighteenth. How numerous and important these 

 discoveries were, is generally known. He ascer- 

 tained the different refrangibility of the rays of 

 light; he made some progress in exploring the 

 principles and laws of colours, v/hieh had been so 

 little understood before his time; he first explained 

 the physical cause, and laid down with mathema- 

 tical precision, the general laws of the reflection 



