Mr. Moon on Fresners Theory of Doithle Refraction. 553 



i^<!> ^•fc'VThe third derivation produced no sensible action on 

 the nfeedle, as was easy to foresee, since the wire of derivation 

 and that of copper of the pair were of the same diameter, and 

 as each one, taken separately, had a conductibility of its own 

 and dimensions sufficient to transmit the whole thermo-elec- 

 tric current. 



■■'"• 79. The method of derivations appears to me to be free 

 from the third objection (72, c), that of the intensities. In fact, 

 the difference in the intensity of the currents which re-united 

 after issuing from one and the same source, and having fol- 

 lowed two entirely similar ways, was capable of being rendered 

 as feeble as possible without any result of interferences. Now 

 analogy being here our sole guide, it is necessary to remember 

 that in wholly similar circumstances, the vibrations of the 

 £Ether which constitute light, and those of the elastic fluids 

 which engender sound, have presented very evident phaeno- 

 mena of mutual destruction*. . . 



LXXVIII. On FresneVs Theory of Double Eefraciion. %R. 

 Moon, M.A., Fellow of Qiieen's College^ Cambridge^ cmd ^ 

 the Cambridge Philosophical Society ■\. 4 



tJFAVING on several previous occasions treated of the 

 ■-■*• theory of unpolarized light, and having, as I trust, ef- 

 fectually exposed the futility of the celebrated hypothesis de- 

 vised by Fresnel for the elucidation of many of the principal 

 phaenomena in that department of optics, I now come to the 

 consideration of the subject of polarized light; upon his treat- 

 ment of which Fresnel's great fame now principally rests, and 

 to whose views in regard to which many of his adherents, who 

 have felt themselves compelled to give up his theory of diffrac- 

 tion, still cling with unshaken fidelity. What my own faith 

 on this subject may be, it is unnecessary at present to disclose 

 further than this, that whether the original idea of transversal 



* In the fundamental experiment of Fresnel, the bundles of light do not 

 necessarily reach the two mirrors under the same incidence, and have not 

 the same intensity when they interfere after reflexion. After M. W. Weber 

 had shown that the surfaces according to which sound disappears around a 

 vibrating diapason are hyperbolically curved, Dr. Kane succeeded, following 

 out an idea of Sir John Herschel, in constructing united tubes, the lengths 

 of which are in the relation of 2 to 3, or of 6 to 7, and which destroy by 

 interference one, in a determined number, of the sounds which is made to 

 pass through them. (Philosophical Magazine, vol. vii. p. 301 ; Poggendorflf's 

 /Innalen der Phj/dk, vol. xxxvii. p. 485.) 



f Communicated by the Author. 

 Phil. Mag. S. 3. No. 183. Suppl. Vol. 27. 2 O j 



