EPOCH OF YOUNG AXD FRESJSTEL. 1X> 



jrystal, on which the velocity of propagation of the luminifercas 

 undulation depends, to be different, in the direction of the crystallo- 

 gTaphic axis, and in the direction of the planes at right angles to this 

 axis ; and from such a, difference, he deduces the existence of spheroi- 

 dal undulations. This suggestion appeared in the Quarterly Review 

 for November, 1809, in a critique upon an attempt of Laplace to 

 account for the same phenomena. Laplace had proposed to reduce 

 the double refraction of such crystals as Iceland spar, to his favorite 

 machinery of forces which are sensible at small distances only. The 

 peculiar forces which produce the effect in this case, he conceives to 

 emanate from the crystallographic axis : so that the velocity of light 

 within the crystal will depend only on the situation of the ray with 

 respect to this axis. But the establishment of this condition is, as 

 Young observes, the main difficulty of the problem. How are we to 

 conceive refracting forces, independent of the surface of the refracting 

 medium, and regulated only by a certain internal line? Moreover, the 

 law of force- which Laplace was obliged to assume, namely, that it 

 \ aried as the square of the sine of the angle which the ray made with 

 the axis, could hardly be reconciled with mechanical principles. In 

 the critique just mentioned, Young appears to feel that the undulatory 

 theory, and perhaps he himself, had not received justice at the ha: 

 of men of science ; he complains that a person so eminent in the 

 world of science as Laplace then was, should employ his influence in 

 propagating error, and should disregard the extraordinary confirmations 

 which the Huyghenian theory had recently received. 



The extension of this view, of the different elasticity of crystals in 

 different directions, to other than uniaxal crystals, Avas a more complex 

 and difficult problem. The general notion was perhaps obvious, after 

 what Young had done ; but its application and verification involved 

 mathematical calculations of great generality, and required also very 

 exact experiments. In fact, this application was not made till Fresnel, 

 a pupil of the Polytechnic School, brought the resources of the modern 

 analysis to bear upon the problem ; till the phenomena of dipolarized 

 light presented the properties of biaxal crystals in a vast variety of 

 forms ; and till the theory received its grand impulse by the combina- 

 tion of the explanation of polarization with the explanation of double 

 refraction. To the history of this last-mentioned great step we now 

 proceed. 



