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XXVI. Remarks on a Paper by Mr. Moon on Fresnel's 

 Theory of Double Refraction*. By Jesuiticus. 



^l^HE hypothesis on which Fresnel's theory of double re- 

 J- fraction is based, is the following: — 



"That the displacement of a molecule of the vibrating me- 

 dium in a crystallized body is resisted by different elastic 

 forces according to the different directions in which the dis- 

 placement takes place." 



This is not a mere speculative hypothesis, but is based on 

 experiment. It is found that glass, possessing only the power 

 of single or ordinary refraction, may be made by the applica- 

 tion of heat, or by mechanical pressure, to possess that of 

 double refraction. 



It is further supposed that the medium is symmetrical with 

 respect to three rectangular axes in space, but, in general, not 

 symmetrical with respect to any other axis through the same 

 origin. These axes are called the axes of elasticity. It is 

 then proved, that if any particle of the aether be suddenly dis- 

 placed, the other particles remaining quiescent, the force of 

 restitution developed by such disturbance will not in general be 

 in the direction of the displacement, but only when such dis- 

 placement is in the direction of the aforesaid axes of elasticity. 

 The elegant demonstration of Smith, quoted by Mr. Moon, 

 is by Mr. Moon's own showing fully adequate to establish the 

 theorem as I have enunciated it, which is doubtless the sense 

 in which Fresnel (the illustrious Fresnel, " whose name is en- 

 rolled amongst those which pass not away,") doubtlessly con- 

 ceived it. 



Any one who understands the subject must at once acknow- 

 ledge that any theory of light must be, to a considerable ex- 

 tent, imaginative; and that theory which can explain the 

 greatest number of facts ought to claim the attention of the 

 philosopher more than any other. It is to this that the undu- 

 latory theory owes its great celebrity, and of all parts of the 

 undulatory theory, that of double refraction is the most extra- 

 ordinary. It ought to be regarded as a stupendous monument 

 of human ingenuity. It must not be forgotten how admirably 

 the properties of uniaxal crystals follow from the general in- 

 vestigation of the biaxal class ; but above all, how from this 

 same investigation, conical and cylindrical refraction were dis- 

 covered by Sir William Hamilton. Such an unexpected re- 

 finement as this, which probably would never have been re- 

 cognised by the mere experimentalist, undirected by the skill 



• Phil. Mag. S. 3. No. 183. vol. xxvii. 



