486 SCIENTIFIC THOUGHT. 



the stereoscope of Wheatstone ; pathological cases, like 

 those of colour-blindness; a host of ingeniously de- 

 vised experiments, as well as the gift of an exception- 

 ally musical ear, all these factors, and innumerable 

 others, contributed to the production of these two 

 monumental works, which form an epoch in the history 

 of science as well as of philosophy and psychology. 

 They form the first magnificent examples of the com- 

 prehensive application of exact methods to phenomena 

 which had before been treated only fragmentarily, and 

 where the influences of taste, fancy, and belief, the 

 vagueness of metaphysics and the difficulties of nomen- 

 clature, had created a confusion which to many must 

 have appeared hopeless. This confusion of language 

 and of terms, of objective observations and subjective 

 fancies, of the data of experience and the prejudices of 

 theory, Helmholtz has done more than any other thinker 

 to unravel. 



' In his two great treatises on the psycho-physics of the 

 Eye and the Ear, of Vision and of Music, he has drawn 

 two elaborate and detailed charts, which for a long time 

 to come will have to be consulted by those who, in the 

 interests of physics, philosophy, or aesthetics, enter into 

 these mysterious domains. Many celebrated theories or 

 definite aspects and lines of reasoning invented by others, 

 his forerunners or contemporaries, were adopted, but 

 mostly with important modifications. It may be of use 

 to enumerate briefly the principal ones, beginning with 

 the most mathematical and exact and ending with the 

 more general and metaphysical. In the beginning of the 

 century Fourier had shown how any forces of motion in 



