CONCLUSION. 231 



not well understood. They are dependent, in some manner, upon molecular 

 arrangement. This is evident from the fact tliat variations rescmblin<>; those 

 which naturally exist in crystals may be produced, as we have seen, in homo- 

 genous bodies, by heat or by the force of pressure, flexure, or torsion. So deli- 

 cate a test does the polariscope furnish of any inequality of temperature, stress, 

 or mechanical force of any kind, that Dr. Brewster has suggested the construc- 

 tion of chromatic thermometers and dynamometers, founded on the principles 

 we have endeavored to unfold, for determining differences of temperature, stress, 

 or pressure too slight to be easily measured by ordinary instruments. 



COi\CLUSION. 



In the review which we have uoav taken of the applications of the doctrine 

 of undulation, we have encountered no optical phenomenon of which this doc- 

 trine does not furnish an explanation ; we have discovered no legitimate deduc- 

 tion from it which has not found its verification in nature. We have seen, on 

 the other hand, that it has served occasionally to point to facts of curious 

 interest previously unknown, which have been subsequently confirmed by the 

 experiments Avhich it has suggested and directed ; experiments which require 

 for their exhibition adjustments so delicate and conditions so difficult to secure, 

 that, but for the clew it has furnished, they would probably have remained forever 

 unknown. This doctrine rises, therefore, above the level of a mer*; hypothesis ; it 

 fulfils every essential condition of a true theory ; it explains all known phenomena; 

 it anticipates the unknown, and its predictions are corroborated by experiment. 



Moreover, the simplicity of the connecting link by which it binds together 

 phenomena the most diverse in their nature, is almost without an example in 

 the history of physical theories. In the words of Fresnel, "in order to calcu- 

 late the so various phenomena of diffraction, those also of the rings produced 

 by thui plates of air or water, or any other refracting medium, refraction itself, 

 in which the ratio of the sine of the incident to the sine of the refracted rays is 

 that of the lengths of the waves in the two media, the colors and the singular 

 modes of polarization presented by crystalline laminre, it is sufficient to know 

 the lengths of undulation of light in the media which it traverses ; this is the 

 sole quantity which it is necessary to borrow from experiment, and it is the 

 basis of all the formula). If we attend to those intimate and multiplied rela- 

 tions which the theory of undulations estabHshes between phenomena the most 

 different, we cannot but be struck at once by its simplicity and its fecundity ; 

 and we are compelled to admit that, even though it had not the advantage over 

 the system of emission of explaining numerous facts absolutely inconceivable in 

 the latter, it would still merit the preference because of the means which it fur- 

 nishes of connecting together all the phenomena of optics and embracing them 

 in general formula?." 



It is not, indeed, to be denied that some embarrassments still attend this theory. 

 There are physicists to whom the phenomena of dispersion still continue to be 

 a stumbling-block, and the differences of opinion which exist in regard to the 

 true relation between the direction of molecular movement in undulation and 

 the plane of polarization of a polarized ray have been pointed out in their 

 proper place; but these difficulties are such as, it is fairly presumable, the 

 further progress of investigation will ultimately clear away, and arc not suffi- 

 ciently serious to impair confidence in the substantial truth of the theory as a 

 whole. At any rate, whether this theory be received or not as a true represen- 

 Uition of the operations of nature in optical phenomena, we are compelled to 

 accept it at present as an instrument for combining and systematizing our knowl- 

 edge of these facts and moulding it into a shape Avorthy of the name of science; 

 since, if we reject this, there is nothing left us on which to fall back which is 

 capable of rendering us the same service. 



