of Light by Coloured Media. 405 



The only difficulty, then, which remains in the application 

 of the undulatory theory to the absorptive phaenomena, is to 

 conceive how a medium (z. e. a combination of aethereal and 

 gross * molecules) can be constituted so as to be transparent, 

 or freely permeable to one ray or system of undulations, and 

 opake, or difficultly permeable to another, differing but little in 

 frequency. Now it is sufficient for our present purpose if, 

 without pretending to analyse 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 experiment, shows to be pos- 

 sible in the case of musical sounds, will hardly be denied to 

 have its analogue or representative among the phaenomena of 

 colour, when referred to the vibrations of an aether. 



An example of an acoustic combination, or compound vibra- 

 ting 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 



Fig. 1. 

 c 



c 

 and symmetrically disposed pipes B C and b c, which reunite 

 again at D d, and there again constitute a single pipe D E, 

 whose direction shall (like A B) bisect the angle between the 

 branches. The branches, however, are of unequal length, 

 the one BCD being longer than the other, by a quantity 

 equal to half the length of the undulation or pulse of the 

 musical note in question. It is evident, then, that if that note 

 be sounded at A, each pulse will subdivide itself at B b 9 and the 

 divided portions will run on along the two branches with equal 

 intensities till they reunite at D d. They will arrive there, how- 



* By gross molecules, or gross bodies, I understand the ponderable con- 

 stituents of the material world, whether solid, liquid, or gaseous ; using the 

 term in contradistinction to aethereal, which has reference to the lumini- 

 ferous aether. 



