28 THE PRINCIPLES OF SCIENCE. 



in which the form and nature of the substance are all 

 important. 



It is interesting to observe how carefully Newton, in 

 his researches on the spectrum, observed and proved the 

 indifference of many circumstances by actual trial. He 

 says b : ' Now the different magnitude of the hole in the 

 window-shut, and different thickness of the prism where 

 the rays passed through it, and different inclinations of 

 the prism to the horizon, made no sensible changes in the 

 length of the image. Neither did the different matter of 

 the prisms make any : for in a vessel made of polished 

 plates of glass cemented together in the shape of a prism, 

 and filled with water, there is the like success of the ex- 

 periment according to the quantity of the refraction/ But 

 in the latter statement, as I shall afterwards remark 

 (vol. ii. p. 42), Newton assumed an indifference which does 

 not exist, and fell into an unfortunate mistake. 



In the science of sound it is shown that the pitch of a 

 sound depends solely upon the number of impulses in a 

 second, and the material exciting those impulses is a 

 matter of perfect indifference. Thus whatever medium, 

 whether air or water, or any gas or liquid, be forced into 

 the Siren, the sound produced is the same ; and the 

 material of which an organ-pipe is constructed does not 

 at all affect the pitch of its sound. 



In the science of statical electricity it is an important 

 circumstance that the interior of a conducting body is a 

 matter of indifference, resting in a neutral state, while the 

 change is confined to the conducting surface. A hollow 

 sphere takes exactly the same charge as a solid sphere of 

 metal. 



Some of Faraday's most elegant and successful re- 

 searches were devoted to the exclusion of conditions 



b 'Opticks,' 3rd edit. p. 25. 



