488 ON THE ABSORPTION OF LIGHT 



fork. Here, then, wq have a case analogous to the easy 

 transmission of a ray of definite colour, accompanied with 

 its gradual extinction, in traversing a considerable thick- 

 ness of the absorbing medium. If we Avould avoid the 

 actual contact of the vibrating systems, we may conceive 

 an arrangement like that here depicted, where, in place 

 of forks, straight bars, disked at both ends and supported 

 at their centres, are used to form the vibrating series. 



(13.) When two disked tuning forks slightly out of 

 unison are opposed to each other, the vibrations of one 

 are still communicated to the other, even when they differ 

 sufficiently to produce audible and pretty rapid beats. 

 But the communication in this case is less complete, and 

 the sound produced feebler, than in that of perfect uni- 

 son, and the degradation of intensity in the communi- 

 cated 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 com- 



