ITS ANALOGY TO SOUND. 



"When light of other colours was thrown upon the glass, a 

 similar system of luminous rings was produced, but it was found 

 in each case that the first ring varied in its diameter according 

 to the colour of the light, and consequently that the amplitude 

 of the waves of lights of different colours is different. It 

 appeared that the waves of red light were the largest ; orange 

 came next to them ; then yellow, green, blue, indigo, and violet, 

 succeeded each other, the waves of each being less than those of 

 the preceding. But the most astonishing part of this most 

 celebrated investigation was the minuteness of these waves. It 

 appeared that the waves of red light were so minute, that 40,000 

 of them would be comprised within an inch, while the waves of 

 violet light, forming the other extreme of the series, were so 

 small, that 60,000 spread over an inch, and the waves of light of 

 other colours were of intermediate magnitudes. 



Thus was discovered the physical cause of the splendour and 

 variety of colours, and a singular and mysterious alliance was 

 developed between colour and sound. Lights are of various 

 hues, according to the magnitude of the pulsations that produce 

 them, exactly as musical sounds vary their tone and pitch 

 according to the magnitude of the aerial pulsations from which 

 they result. 



But this is not all. The alliance between sound and light 

 does not terminate here. We have only spoken of the amplitude 

 of the luminous waves, and have shown that it determines the 

 tints of colours. What are we to say for the altitudes of the 

 waves ? Here, again, is another link of kindred between the 

 eye and the ear. As the altitude of sonorous waves determines 

 the loudness of the sounds, so the altitude of luminous waves 

 determines the intensity or brightness of the colour. 



There is one step more in the series of wondrous results which 

 these memorable investigations have unfolded. As the percep- 

 tion of sound is produced by the tympanum of the ear vibrating 

 in sympathetic accordance with the pulsations of the air pro- 

 duced by the sounding body, so the perception of light and colour 

 is produced by similar pulsations of the membrane of the eye 

 vibrating in accordance with ethereal pulsations propagated 

 from the visible object. As in the case of the ear, the rigour of 

 scientific investigation requires us to estimate the rate of the 

 pulsation of the tympanum corresponding to each particular 

 note, so in the case of light are we required to count the vibra- 

 tions of the retina corresponding to every tint and colour. It 

 may well be asked, in some spirit of incredulity, how the solution 

 of such a problem could be hoped for ; yet, as we shall now see, 

 nothing can be more simple and obvious. 



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