of Light by Coloured Media, 411 



their mode of vibration, and even different disposals of their 

 nodal lines and surfaces, according to the different directions 

 in which undulations may traverse them, and which may not 

 impossibly be found to render an account of the change of tint 

 of such media according to the direction of the rays in their 

 interior, as well as of the different tints and intensities of their 

 oppositely polarized pencils; of which latter class of phaenornena, 

 however, I shall immediately have occasion to speak further. 



But as my present object is merely to throw out, as a sub- 

 ject for examination, a hint of a possible explanation of the 

 phaenornena of absorption, on the undulatory theory, I shall not 

 now pursue its application into any detail, nor attempt the 

 further development of particular laws of structure competent 

 to apply to this or that phenomenon. I will, however, men- 

 tion one or two facts in acoustics which appear to me strongly 

 illustrative of corresponding phaenornena in the propagation of 

 light. The first of these is the impeded propagation of sound 

 in a mixture of gases differing much in elasticity as compared 

 with their density. The late Sir J. Leslie's experiments on the 

 transmission of sound through mixtures of hydrogen with at- 

 mospheric air sufficiently establish this remarkable effect. It 

 would be desirable to prosecute those experiments in larger 

 detail, but hitherto I am not aware of anybody having ever 

 repeated them. It would be interesting, for instance, to in- 

 quire whether the impediment offered by such a mixture of 

 gases be the same for all 'pitches of a musical note, or not ; and 

 how far this phaenomenon might be imitated by mixing actual 

 dust of a uniform size of particle, such as the dust of Lycoper- 

 don, &c, or aqueous fog, and how far such mixture would 

 affect unequally sounds of different pitches. 



The other fact in the science of acoustics which I would no- 

 tice as illustrative of a corresponding phenomenon in photo- 

 logy, is one observed by Mr. Wheatstone, which I have his 

 permission to mention. In attempting to propagate vibrations 

 along wires, rods, &c, to great distances, he was led to remark 

 a very great difference in respect of facility of propagation 

 between vibrations longitudinal and transverse to the general 

 direction of propagation. The former were readily conveyed 

 with almost undiminished intensity to any distance ; the latter 

 were carried off so rapidly by the air, as to be incapable of be- 

 ing transmitted with any considerable intensity to even mode- 

 rate distances. This strikes me as obviously analogous to the 

 ready transmissibility of a ray polarized in one certain direc- 

 tion, through a tourmaline or other absorbing doubly defract- 

 ing crystal, while the oppositely polarized ray (whose vibra- 

 tions are rectangular to those of the first) is rapidly absorbed 



3G2 



