PROPAGATION OF LIGHT. 17 



C19) We have hitherto considered the propagation of 

 vibratory movement without reference to any diversity of 

 its nature. It is obvious, however, that vibrations may 

 differ from one another in two particulars, namely, in the 

 space of vibration, aud in the time. In the aerial pulses the 

 amplitude of the vibration determines the loud ness of the 

 sound ; and the frequency of the pulses, or the time of vibra- 

 tion, determines its note. In like manner, the amplitude of 

 the ethereal vibrations determines the intensity of the light ; 

 and their frequency, or the period of vibration, determines 

 the colour. Thus, two lights may differ from one another in 

 intensity and colour, the former depending (according to the 

 wave-theory) on the space of vibration, and the latter on the 

 time. 



But although the intensity of the light is obviously depen- 

 dent on the amplitude of the vibration, yet it does not appear, 

 a priori, by what power of the amplitude it is to be repre- 

 sented. In fact, we must define what we mean by a double, 

 triple, &c, quantity of light, before we can know how that 

 quantity is to be mathematically measured. If then we say 

 that a double light is the sum of the lights produced by two 

 luminous origins of equal intensity, placed close together, it 

 is easy to prove that the quantity of light, in general, is 

 measured by the square of the amplitude of the vibration. 

 From this it follows that the intensity of the light diverging 

 from any luminous origin must decrease inversely as the 

 square of the distance ; for, from the laws of wave propaga- 

 tion, it appears that the space of vibration diminishes in the 

 inverse simple ratio of the distance. Thus the known law 

 of the variation of the intensity of light is deduced from the 

 principles of undulatory propagation. 



(20) The colour of the light (it has been said) depends 

 on the number of impulses which the nerves of the eye receive, 



[UFI7BESITT 



