Vibrations of a Polarized Ray, IQS 



turned aside, or diffracted, according to laws which the wave- 

 theory has explained. Now Professor Stokes has shown that, 

 when the incident light is polarized, the plane of vibration of the 

 diffracted ray must differ from that of the incident, the two 

 planes being connected by a very simple relation. It only re- 

 mained, therefore, for observation to determine whether the 

 planes of polarization of the incident and refracted rays were 

 similarly related, or not. The experiment was undertaken by 

 Professor Stokes himself, and he has inferred from it that the 

 original hypothesis of Fresnel is the true one ; but, as an oppo- 

 site result has been obtained by M. Holtzmann on repeating 

 the experiment, the question must be regarded as still undeter- 

 mined. The difference in the experimental results is ascribed 

 by Professor Stokes to the difference in the nature of the gra- 

 tings employed, the substance of the diffracting body being sup- 

 posed to exert an effect upon the polarization of light, which is 

 diffracted by it under a great obliquity. I learn from Professor 

 Stokes that he proposes to resume the experimental inquiry, and 

 to test this supposition by employing gratings of various sub- 

 stances. If the conjecture should prove to be well founded, it 

 will, unfortunately, greatly complicate the dynamical theory of 

 light." 



It must be admitted that the question thus presents a very 

 unsatisfactory aspect. Considering the amount of talent that 

 has been brought to bear upon it, and the great length of time 

 during which it has been under discussion, I feel confirmed by 

 the above statements in the opinion I have long entertained, 

 that there is no foundation for the oscillatory theory of light. 

 The having recom-se to a diffraction experiment to settle a point 

 which it is scarcely possible that a good theory applied to any 

 instance of polarization could leave imdecided, appears to be 

 only a symptom of a failing theory. It must be borne in mind 

 that in this theory the constitution of the luminiferous medium 

 is entirely hypothetical, and that the hypotheses were framed 

 expressly to account for transverse vibrations, which were found 

 by experiment to be necessary to explain certain phsenomena of 

 light. It is not likely, I think, that this course would have been 

 taken, if it had been known in Fresnel's time that the free vibra- 

 tions of a medium constituted like air, are both longitudinal and 

 transversal, as I have shown to be the case on the principles of 

 hydrodynamics. Since, therefore, the hypothetical medium of 

 the oscillatory theory has hitherto failed to determine in what 

 direction the vibrations of a polarized ray take place, it seems 

 to be the proper time to inquire what answer the question 

 receives from the undulatory theory of light treated according 

 to hydrodynamical principles. This is what I now propose to do. 



