AND THE THEORY OF LIGHT. 53 



tions raised bj the opposing theories, my intention being 

 to keep one point, and one only, in view ; viz. to show by 

 reasoning and experiment the necessity of introducing a 

 negative term in the explanation of the phenomena of 

 colour, and to follow out the conclusions to be derived 

 from the experiments. It may therefore be advisable to 

 state the argument in favour of a negative term derivable 

 from the ether itself before entering on the experimental 

 part of my subject. 



j^. It has been customary to investigate the theory of 

 light by the analogy of sound, sound being formed in an 

 ambient fluid analogous to the supposed luminous ether; 

 that is, both being produced by a similar physical process, 

 sound and light are supposed to follow the same laws. 

 They may obey the same laws ; this I am not disputing ; 

 but these laws must be investigated on different prin- 

 ciples. For allowing that they do obey the same physical 

 laws, still at the commencement these laws operate in a 

 difiFerent manner. It would, therefore, not be philoso- 

 phical to assume a complete analogy. For in the theory 

 of light, motion or an undulating ether is given us ; we 

 have therefore to investigate the various laws by which 

 motion is diminished or modified so as to produce variety 

 of colour. In the theory of sound, silence or rest is given 

 us ; we have therefore to investigate the laws by which 

 motion is produced in a circumambient fluid so as to give 

 rise to variety of sound or tone. The one deals with an 

 analytical process, the other with a synthetical process. 

 One process consists in the resolution of force, the other 

 in the generation of force. Sound is created by a solid 

 body communicating its vibrations to the surrounding 

 atmosphere ; colour is produced by a solid body intercept- 

 ing or reducing the vibrations of a vibrating ether. The 

 one is consequently the inverse process of the other, and 

 it should not be assumed that they follow the same laws 



