ON LIGHT. 275 



end to end of a stretched cord, or to the waves which ap- 

 pear to travel along the surface of water ; though in reality 

 such a wave is only an advancing form, the real move- 

 ment of the watery particles being vertically up and down. 

 Colour in this view of the subject is analogous to tone, or 

 pitch, in music (if it be supposed to depend solely on re- 

 frangibility). As the frequency of the vibrations which 

 reach the ear from a sounding-string determines the pitch 

 .of the musical note it yields, so the frequency of the un^ 

 dulations of this elastic medium or luminiferous " ether," 

 as it is called, determines to the nerves of the eye the 

 colour of the light. Or in that view of colour which 

 considers all but three primary hues composite, it must 

 on this theory be assimilated to a difference analogous to 

 quality in a musical tone as, for instance, between the 

 sounds of a violin, a flute, and a trumpet, only much 

 more decided and strongly characterized. 



(59.) As sound spreads through the air with equal rapid- 

 ity in all directions, and may be considered as propagated 

 from its origin as a spherical shell continually enlarging, 

 so in this theory must light be regarded as the move- 

 ment of a WAVE in the ether, running out spherically in 

 all directions from the luminous point, whose situation 

 with respect to the eye, or to any other point on which 

 the wave may strike, is judged of as the centre of the 

 sphere i.e., as lying in a line perpendicular to its sur- 

 face. A ray of light then, in this theory, is a purely 

 imaginary line from such point, perpendicular to the 

 general surface, or front of the wave, and has no other 

 meaning. The wave, not the ray, is the primary object 



