420 British Association. 



the inference, that the diurnal variation is a direct effect of solar action, 

 and not a result of its thermic agency* * 



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 clUpticaUy 

 polarized; and the phenomenon is explained on the principles of the 

 •wave-theory, by the assumption that the vibration of the ether undergoes 

 a change of phase at the instant of reflexion, the amount of which is de- 

 pendent 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 Prof. Powell extended the property to all bodies having a 

 high refractive powder. But it was not until lately, that M. Jamin proved 

 that thei-e is no disimction in this respect between transparent and metal- 

 lic bodies; that all bodies transform plane-polarized into ellipticallj-po- 

 larized light, and impress a change of phase at the moment of reflexion. 

 Prof. Haughton has followed up the researches of M. Jamin, and estab- 

 lished the existence of circularly-polarized light by reflexion from trans- 

 parent surfaces. 



The theoretical investigations connected with this subject afford a re- 

 markable illustration of one of those impediments to the progress of 

 Natural Philosophy which Bacon has put in tlie foremost place among 

 his examples of the Idola — I mean the tendency of the human mind to 

 fiuppose a greater simplicity and uniformity in nature than exists there. 

 The phenomena of polarization compel us to admit that the sensible lu- 

 iniuous vibrations are transversal^ or in the plane of the wave itself; and 

 it was naturally supposed by Fresnel, and after hira by M'CulIagh 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 j^hat^e 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 

 W'ave-theory of light, respecting which opposite assumptions have been 

 inade. The vibrations of a polarized ray are all parallel to a fixed direc- 

 tion in the plane of the wave ; but that direction may be dih^r parallel 

 ^r 2yerpendicular to the plane of polarization. In the original theorjr of 

 Fresnel, the latter was assumed to be the fact; and in this assumption 

 Presnel has been followed by Cauchy. In the modified theories of 

 M'CulIagh 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 compensated, as respects the results, by other differences m 

 their hypothetical principles; and both of them 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 Prof. 

 Stotes recently, in applying the dynamical theory of light to other classes 

 of phenomena, found one in which the effects should differ on the two 

 assumptions. When light is transmitted through a fine grating, it is 

 turned aside, or diffracted, aecordino- to laws which the wave-theory has 



% 



