408 



Sir John F. W. Herschel on the Absorption 



ries of such forks and loops arranged 

 as in fig. 2, and let the first, A, be 

 maintained in vibration by any ex- 

 citing cause, as, for instance, by 

 causing to sound a musical note op- 

 posite to its disk A, in unison with its 

 pitch. The vibrations so excited will, 

 as is evident, run along the whole line, 

 though with diminishing intensity, to 

 the last fork. Here, then, we have a 

 case analogous to the easy transmis- 

 sion of a ray of definite colour, accom- 

 panied with its gradual extinction, in 

 traversing a considerable thickness of 

 the absorbing medium. If we would 

 avoid the actual contact of the vi- 

 brating systems, we may conceive an arrangement like that 

 in fig. 3, where, in place of forks, straight bars, disked at both 



Fig. 3. 



ends and supported at their nodal points, are used to form the 

 vibrating series. 



When two disked tuning forks slightly out of unison are 

 opposed to each other, the vibrations of one are still communi- 

 cated to the other, even when they differ sufficiently to pro- 

 duce audible and pretty rapid beats. But the communication 

 in this case is less complete, and the sound produced feebler, 

 than in that of perfect unison, and the degradation of intensity 

 in the communicated sound is very rapid as the forks recede 

 from unison. We have here a fact analogous to the appear- 

 ance of a bright line in the spectrum situated between dark 

 spaces, and as it is not difficult to imagine combinations of the 

 nature above mentioned, in which several different notes shall 

 be transmitted, while the intermediate one, finding no unisons, 

 or near approaches to unison in the systems established, shall 



