FOR THE ADVANCEMENT OF SCIENCE, 1857. 497 



and Mr. Dale and Professor Powell extended the property to all 

 bodies having a high refractive power. But it was not until 

 lately that M. Jamin proved that there is no distinction, in this 

 respect, between transparent and metallic bodies; and that all 

 bodies transform plane-polarized into elliptically-polarized light, 

 and impress a change of phase at the moment of reflexion. 

 Professor Haughton has followed up the researches of M. Jamin, 

 and established the existence of circularly-polarized light by reflex- 

 ion from transparent surfaces. 



The theoretical investigations connected with this subject afford 

 a remarkable illustration of one of those impediments to the pro- 

 gress of natural philosophy, which Bacon has put in the foremost 

 place among his examples of the Idola : I mean the tendency of 

 the human mind to suppose a greater simplicity and uniformity 

 in nature than exists there. The phenomena of polarization 

 compel us to admit that the sensible luminous vibrations are 

 transversal, or in the plane of the wave itself ; and it was naturally 

 supposed by Fresnel, and after him by M'Cullagh and Neumann, 

 either that no normal vibrations were propagated, or that, if they 

 were, they were unconnected with the phenomena 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. 



Attention has been lately recalled to a fundamental position of 

 the wave-theory of light, respecting which opposite assumptions 

 have been made. The vibrations of a polarized ray are all parallel 

 to a fixed direction in the plane of the wave ; but that direction 

 may be either parallel, or perpendicular, to the plane of polarization. 

 In the original theory of Fresnel the latter was assumed to be the 

 fact ; and in this assumption Fresnel has been followed by Cauchy. 

 In the modified theories of M'Cullagh and Neumann, on the 

 other hand, the vibrations are supposed to be parallel to the plane 

 of polarization. This opposition of the two theories was compen- 

 sated, as respects the results, by other differences in their hypothe- 

 tical principles; and both of them have led to conclusions which 

 observation has verified. There seemed, therefore, to be no means 

 left to the theorist to decide between these conflicting hypotheses, 

 until Professor Stokes, recently, in applying the dynamical theory 

 of light to other classes of phenomena, found one in wlii >h tlm 

 effects should differ on the two assumptions. When light is 



2K 



