468 



BRITISH SCIENTIFIC ASSOCIATION. 



its two leading departments. Land observations exist in great numbers. In 

 Prussia, in Russia, in Austria, and in Belgium, such observations are organized 

 under Government direction, or at least with Government support. In other parts 

 of Europe, as in Britain, the labour is left to individuals or scientific Societies. 

 What is needed is to give unity to these isolated labours — to connect them with 

 one another, and with the results obtained at sea ; and the first step to this seems 

 to be to give them, in each country, that permanence and uniformity of system 

 which cau only be insured in measures adopted by the State. ' 



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" 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 assumption that the 

 vibration of the ether undergoes a change of phase at the instant of reflexion, the 

 amount of which is dependent on its direction and on the angle of incidence. This 

 supposed distinction, however, was soon found notto 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 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 elliptically-polarized light, and impress a change of phase at the 

 moment of reflexion. Prof. Haughton has followed up the researches of M. 

 Jamin, and established the existence of circularly polarized light by reflexion from 

 transparent surfaces. 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 tendency of the human mind to suppose a greater 

 simplicity and uniformity in nature than exists there. The phenomena of polari- 

 zation 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 McCullagh 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 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 perpendicidar 

 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 McCullagh and Neumann, on the other hand, 

 the vibrations are supposed to be parallel to the plane of polarization. This 

 opposition of the hvo 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, therefore, to be no means left to 

 the theorist to decide between these conflicting hypotheses until Prof. Stokes 

 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 



