466 JAMES CLERK MAXWELL. 



6. Faraday's Lines of Force, and Maxwell's Theory of 

 the Electro-magnetic Field, including the Electro-magnetic 

 Theory of Light and other investigations in Electricity. 



7. Molecular Physics. 



1. The subject of colour- vision attracted Clerk Maxwell's 

 attention at an early period. In dealing with phenomena 

 of this class, we must remember that it is necessary to dis- 

 tinguish between the sensation itself and its physical cause, 

 or between the subjective and objective aspects of the same 

 phenomenon. Thus a pure musical tone consists of a regular 

 succession of similar vibrations, and two tones may differ in 

 the extent of these vibrations, and in the number which 

 take place in a second. Corresponding to these physical 

 differences, are experienced differences in the intensity 

 or loudness, and the pitch of the note. Light, like sound, 

 consists objectively of certain periodic disturbances or vibra- 

 tions of a medium, but differs from sound in the character 

 of the motion, the nature of the medium which transmits it, 

 and the number of vibrations which take place in a second. 

 The simplest kind of light consists, like a pure tone in 

 sound, of a regular succession of similar vibrations, the 

 extent of which determines the intensity of the light, while 

 their rapidity corresponds to the colour sensation produced. 

 The constitution of ordinary white light is much more com- 

 plicated. Newton allowed a beam of sunlight to pass 

 through a prism, and then to fall upon a screen, when, in- 

 stead of a white patch of light, he obtained a spectrum, or 

 continuous band of colour, varying from crimson through 

 scarlet, orange, yellow, green, blue, to violet. Rays corre- 

 sponding to this infinite variety of colour must, therefore, 

 exist together in white solar light, and these rays differ 

 physically from one another in the rapidity of the vibrations 

 of which they consist ; the deep crimson corresponding to 

 less than 400,000,000,000,000 vibrations per second, and 

 the extreme violet to more than 700,000,000,000,000, the 

 length of a wave in air being, in the first case, about ^--gyjj<j <y 

 of an inch, and, in the second case, about -^ of an inch. 



