286 Prof. Challis on the Theory of Elliptically-polarised Light, 



the direction of the vibrations should eventually be determined 

 by having recourse to phseuomena of diffraction, the case of the 

 oscillatory theory will not thereby be improved, because the 

 usual mathematical treatment of that class of phrenomena is 

 more accordant with the hypothesis of a continuous fluid medium 

 than with any other. I proceed now, with the same intention, 

 to discuss the oscillatory theory of clliptically-polarized light ; 

 and in order to point out the actual state of this theory, I shall 

 again refer to the able and useful address of Dr. Lloyd at the 

 Dublin Meeting of the British Association in 1857. 



After informing us of recent experiments by which it has been 

 established that by reflexion "all bodies transform plane-polarized 

 light into clliptically-polarized light, and impress a change of 

 phase at the moment of reflexion," Dr. Lloyd adds as follows : — 



"The theoretical investigations connected with this subject 

 afford a remarkable illustration of one of those impediments to 

 the progress of natural philosophy which Bacon has put in the 

 foremost place among his examples of the Idola : I mean the ten- 

 dency of the human mind to suppose a greater simplicity and 

 uniformity in natm-e than exists there. The phseuomena of po- 

 larization compel us to admit that the sensible luminous vibra- 

 tions are transversal, or in the plane of the wave itself; and it 

 was naturally supposed by Fresnel, and after him by MacCullagh 

 and Neumann, either that no normal vibrations were propagated, 

 or that, if they were, they were unconnected with the phseno- 

 mena of light. We now learn that it is by them that the phase 

 is modified in the act of reflexion, and that, consequently, no 

 dynamical theory which neglects them, or sets them aside, can 

 be complete." 



I believe I am correct in asserting that mathematicians who 

 have treated of the hypothetical medium from which transverse 

 vibrations have been deduced, have shown that the constitution 

 (isotrope) which conducts to transverse vibrations excludes any 

 change of density, and therefore excludes any normal vibrations 

 that can have connexion with the phsenomena of light. Hence 

 if experiment proves, as above stated, that normal vibrations 

 have a real efiect on phsenomena of light, it follows that the hy- 

 pothetical constitution of the luminiferous medium is contradicted 

 by fact, and must therefore, by an acknowledged rule of philo- 

 sophizing, be abandoned. It will be necessary, if the same 

 course of investigation be persisted in, to invent another consti- 

 tution of the medium, in order to meet the case of the coexist- 

 ence of normal with transverse vibrations. As I am not aware 

 that the molecular constitution of a medium having this property 

 has hitherto been indicated, I think that I shall be justified in 

 the endeavour to call attention to the theory of luminous rays, 



