REPORT ON PHYSICAL OPTICS. 319 



Specific actions, varying in the most abrupt and irregular man- 

 ner with the refrangibility of the ray *. 



The absorption of light, and the opacity of bodies, were long 

 since urged by Halley as difficulties in the wave-theory. The 

 ether is supposed to penetrate all bodies freely, and why not 

 also the undulatory motion in which light consists ? To this 

 difficulty we find a full and complete solution in the principle of 

 interference. When a wave enters a discontinuous substance, 

 it will be broken up, and its parts undergo continued subdivision 

 by internal reflexions ; so that when these parts reach the 

 second surface of the body, they are found in every possible 

 phase, and must destroy one another by interference. The 

 phenomenon, as has been observed by Sir John Herschel, is 

 analogous to the impeded propagation of sound in a mixture of 

 gases differing much in elasticity as compared with their density. 



The same writer has given an ingenious and natural account 

 of the absorption of specific rays on the principles of the wave- 

 theory, in a paper read before the Association last year f . He 

 considers the molecules of the body and those of the ether as 

 forming, conjointly, compound vibrating systems, which are 

 more disposed to transmit vibrations of some determinate period 

 than others. Other vibrations, however, not in unison with 

 these systems, may be propagated through them. These/orcerf 

 vibrations, as he calls them, will be obstructed in their pi-ogress, 

 and their amplitudes diminished by the mutual influence of the 

 motions of the parts of the systems ; and he shows that it is 

 possible to conceive systems, which will be wholly impervious 

 to a vibration of a particular period, while they freely transmit 

 others not differing from them materially in their frequency |. 

 But these important and interesting speculations, it must be 

 remembered, are advanced by their author solely with the view 

 of removing an imagined inconsistency between the phenomena 

 of absorption and the mechanical laws of vibratory movement. 



* See Sir David Brewster's Report on Optics. 



f " On the Absorption of Light by coloured Media, viewed in connexion with 

 the Undulatory Theory," Phil. Mag., Third Series, vol. iii. 



t An interesting interference experiment, similar in some respects to that 

 indicated by Sir John Herschel in this paper, has been recently made by Mr. 

 Kane. A compound tube, whose branches of 9 and 13| inches united at the 

 two extremities, was made to sound by the languette of an organ pipe. Each 

 of the tubes, separately, gave its own fundamental note, and all its harmonics; 

 and when a free communication was opened between them, the system gave all 

 the notes of the two series, with the exception of those whose waves were in 

 complete discordance. Thus the fundamental note of the short tube was stopped 

 altogether, while its octave was given with remarkable clearness ; the two 

 waves being in complete discordance in the former case, and in complete ac- 

 cordance in the latter. 



