XIV. 



A COMPARISON OF THE ELASTIC AND THE ELECTRICAL 

 THEORIES OF LIGHT WITH RESPECT TO THE LAW OF 

 DOUBLE REFRACTION AND THE DISPERSION OF COLORS. 



[American Journal of Science, ser. 3, vol. xxxv, pp. 467-475, June, 1888.] 



IT is claimed for the electrical * theory of light that it is free from 

 serious difficulties, which beset the explanation of the phenomena of 

 light by the dynamics of elastic solids. Just what these difficulties 

 are, and why they do not occur in the explanation of the same 

 phenomena by the dynamics of electricity, has not perhaps been 

 shown with all the simplicity and generality which might be desired. 

 Such a treatment of the subject is however the more necessary on 

 account of the ever-increasing bulk of the literature on either side, 

 and the confusing multiplicity of the elastic theories. It is the object 

 of this paper to supply this want, so far as respects the propagation 

 of plane waves in transparent and sensibly homogeneous media. The 

 simplicity of this part of the subject renders it .appropriate for the 

 first test of any optical theory, while the precision of which the 

 experimental determinations are capable, renders the test extremely 

 rigorous. 



It is moreover, as the writer believes, an appropriate time for the 

 discussion proposed, since on one hand the experimental verification 

 of Fresnel's Law has recently been carried to a degree of precision far 

 exceeding anything which we have had before,! and on the other, the 



* The term electrical seems the most simple and appropriate to describe that theory of 

 light which makes it consist in electrical motions. The cases in which any distinctively 

 magnetic action is involved in the phenomena of light are so exceptional, that it is 

 difficult to see any sufficient reason why the general theory should be called electro- 

 magnetic, unless we are to call all phenomena electromagnetic which depend on the 

 motions of electricity. 



t In the recent experiments of Professor Hastings relating to the index of refraction of 

 the extraordinary ray in Iceland spar for the spectral line D 2 and a wave-normal inclined 

 at about 31 to the optic axis, the difference between the observed and the calculated 

 values was only two or three units in the sixth decimal place (in the seventh significant 

 figure), which was about the probable error of the determinations. See Am. Jour. Sci. 

 ser. 3, vol. xxxv, p. 60. 



