Intelligence and Miscellaneous Articles. 243 



remembered was held by Maccullagh and Neuman, and at one time 

 by Cauchy ; the latter is the view taken by Fresnel, Cauchy. Beer 

 and the majority of physicists who have written upon the subject. 

 Our readers will remember that the question, considered from the 

 mathematical point of view, amounts to this. Is the density of the 

 £ether to be considered constant and its elasticity variable ; or is the 

 elasticity to be considered constant and the density variable ? The 

 former supposition leads to the conclusion that the vibrations are at 

 right angles to the plane of polarization ; the latter that they are in 

 this plane. It is only an appeal to experiment which can decide 

 the question, or rather it is only this appeal which can throw the 

 weight of probability upon the one side or the other. Haidinger 

 supports Fresnel's view, and bases his reasoning upon the phseno- 

 mena of pleochroism in doubly refracting crystals. We shall simply 

 translate the author's succinct expression of his own argument. 



I. Let the object be a dichroous crystal and let equal thicknesses 

 of its substance be investigated. 



II. The following positions are considered as demonstrated: — 



a. The vibrations of the luminiferous aether arc transverse, 



b. To the same colours belong equal wave-lengths ; to different 

 colours diiferent wave-lengths. 



III. Mode of investigation. 



(1.) Observation. — In the horizontal zone (of a uniaxial crystal) 

 whose edges are parallel to the axis in all azimuths, one ray or 

 bundle of rays (an image of the dichroscopic lens or of any doubly 

 refracting prism), viz. the ordinary ray, is polarized parallel to the 

 axis with the colour A, and one ray, or bundle of rays, the extraordi- 

 nary ray, is polarized perpendicular to the axis with the colour B. 



Inference. — The vibrations are either perpendicular to the plane 

 of polarization or in this plane. 



Hypothesis. 



1. The vibrations are perpen- 2. Tlie vibrations are in the 



dicular to the plane of polarization, plane of polarization. 



Consequences. 



1. The direction of the vibra- 1. The direction of the vibra- 

 tions of the ordinary ray is per- tions of the ordinary ray lies in 

 pendicular to its plane. There its plane. For all azimuths there 

 are an infinite number of such is but one such direction of vi- 

 directions ; they are perpendicu- bration. It is in the direction of 

 lar to the axis. the axis. 



2. To one colour A or wave- 2. To one colour A or wave- 

 length belongs an infinite num- length belongs only one direc- 

 ber of directions of vibration, tion of vibration. 



but in as many different planes 

 of polarization. 



3. To an infinite number of 3. To an infinite number of 



