288 Prof. Challis on the Theory of Elliptically-polarized Light. 



be so short that several hundred series enter the eye in every 

 second of time." All that I am concerned with in this state- 

 ment is, the evidence it gives that a separate hypothesis, in no 

 manner related to the original hypothetical medium from which 

 alone transverse vibrations were derived, has been found neces- 

 sary to distinguish between common light and elliptically-polar- 

 ized light. According to strict deduction from the hypothetical 

 medium there is no such distinction; and consequently, as there is a 

 distinction in fact, the same regida philosophandi which I referred 

 to in speaking of normal vibrations, requires that the hypothe- 

 tical medium be abandoned. Mr. Airy's supplementary hypo- 

 thesis involves the necessity of indicating the constitution of a 

 medium which has the property of producing these alternating 

 series of transverse vibrations ; but as this has not been done, I 

 beg to call attention to the very simple account of the different 

 kinds of light which is given by the hydrodynamical theory. In 

 this theory common light consists of quaquaversus rectilinear 

 transverse vibrations equal in all directions from the axis of the 

 ray, plane-polarized light, of rectilinear transverse vibrations 

 parallel to a plane, circularly and elliptically-polarized light of 

 circular and elliptical transverse vibrations. 



In a paper*" On the Composition and Resolution of Streams 

 of Polarized Light" (Camb. Phil. Trans, vol. ix. part 3), Prof. 

 Stokes has adopted Mr. Airy's hypothesis, and argued that the 

 transition from one series to the next may be gradual ; but he 

 has not indicated a constitution of the luminiferous medium 

 which will help us to conceive of a cause for such transitions. 



The success with which the oscillatory theory of light accounts 

 for the phsenomena of double refraction has been appealed to as 

 evidence of its truth. But it is to be observed that there are two 

 distinct classes of facts to be explained in physical optics ; the 

 nature and qualities of luminous rays considered by themselves, 

 and the laws according to which they comport themselves when 

 they come into contact with visible media. The latter class of 

 phsenomena are the more complex ; and the treatment of them 

 necessitates additional hypotheses i-especting the molecular con- 

 stitution of these media. In FresneFs theory of double refrac- 

 tion, the hypotheses relating to the Eether and the refracting 

 media are so blended, that that theory is inappropriate to be a 

 test of the constitution of the setherial medium itself. In the 

 Cambridge Philosophical Transactions, vol. viii. part 4, I have 

 obtained the known equation of the wave-surface, on the hypo- 

 thesis that the ajthcr is a continuous medium of variable density 

 and pressure, joined with certain other hypotheses relating ex- 

 clusively to the molecular constitution of the doubly refracting 

 substance. 



