BY COLOURED MEDIA. 



4S3 



meable to another, differing but little in frequency. Now 

 it is sufficient for our present purpose if, without pretend- 

 ing to analyze the actual structure of any optical medium, 

 we can indicate structures and combinations in which 

 air, in lieu of the aether, is the undulating medium, and 

 which shall be either incapable of transmitting a musical 

 sound of a given pitch, or shall transmit it much less 

 readily than sounds of any other pitch, even those nearly 

 adjacent to it. For that which experiment, or theory so 

 well grounded as to be equally convincing with experi- 

 ment, shows to be possible in the case of musical sounds, 

 will hardly be denied to have its analogue or represen- 

 tative among the phenomena of colour, when referred to 

 the vibrations of an aether. 



(9.) An example of an acoustic combination, or com- 

 pound vibrating system, incapable of transmitting a 



musical sound of a given pitch, is furnished by the pipe 

 A E, which, after proceeding singly a certain length A B, 

 at B branches off into two equal and symmetrically dis- 

 posed pipes B C and b c, which reunite again at D d, and 

 there again constitute a single pipe D E, whose direction 



