210 ANNUAL OF SCIENTIFIC DISCOVERY. 



ture. A similar remark might be applied to many other important atmos- 

 pheric phenomena, and these lines would also serve to indicate more clearly, 

 or to lessen the possibility of the supposed connection between terrestrial 

 ^magnetism and terrestrial temperature. 



RECENT DISCOVERIES IN RELATION TO LIGHT. 



The most important of the recent additions to the theory of Light have 

 been those made by M. Jamin. It has been long known that metals differed 

 from transparent bodies, in their action on light, in this, that plane-polarized 

 light reflected from their surfaces became elliptically polarized; and the 

 phenomenon is explained on the principles of the wave-theory, by the rf- 

 sumption that the vibration of the ether undergoes a change of phase at the 

 instant of reflection, the amount of which is dependent on its direction and 

 on the angle of incidence. This supposed distinction, however, was soon 

 found not to be absolute. Mr. Airy showed that diamond reflected light in a 

 manner similar to metals ; 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 ; that all bodies transform plane- 

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

 the moment of reflection. Prof. Haughton has followed up the researches 

 of M. Jamin, and established the existence of circularly-polarized light by 

 reflection from transparent surfaces. 



The theoretical investigations connected with this subject afford a i-emark- 

 able illustration of one of those impediments to the progress of Natural 

 Phik^ophy, 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 had no relation to the 

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

 modified in the act of reflection; and that, consequently, no dynamical 

 theory which neglects them, or sets them aside, can be complete. 



Attention lias 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 polari- 

 zation. This opposition of the two theories was compensated, as respects 

 the results, by other differences in their hypothetical principles ; and both 

 of them led to conclusions which observation has verified. There seemed, 



