460 



theories already before the scientific public, which fulfil this 

 essential condition, and I now publish a sixth mechanical 

 theory, not because I think it superior to its predecessors, 

 but in order, if possible, to direct attention to the unscientific 

 state in which the question rests. It may be useful, with 

 this view, to mention the various theories, which I shall do 

 in chronological order : — First, Fresnel's theory ; second, 

 M. Cauchy's theory, deduced from the mathematical equations 

 of motion of a system of attracting and repelling molecules ; 

 third and fourth, two theories of Mr. George Green, published 

 in 1839 ; fifth, the late Professor Mac Cullagh's theory, pub- 

 lished in the same year. To these may be added the theory 

 now published. The existence oijive rival theories is a for- 

 midable objection to each of the six, and until this objection 

 is removed, none can claim to be the theory of Light. 



" The experimenta crucis must be sought for in the laws 

 of reflexion and refraction, as I have shown in my former 

 paper. I am at present engaged in the investigation of the 

 laws of reflexion, with the view of testing by experiment, if 

 possible, the six theories of light. It may be interesting to 

 observe, with respect to the direction of vibration, that in 

 M. Cauchy's theory the vibrations are neither normal nor 

 transversal, that in Fresnel's and Mr. Green's second theory, 

 the vibrations are perpendicular to the plane of polarization, 

 and that in Mr. Green's first theory, Professor Mac Cullagh's, 

 and my own, the vibrations are parallel to the plane of po- 

 larization." 



The Rev. Samuel Haughton communicated also the fol- 

 lowing note on the function peculiar to a system of attracting 

 and repelling molecules. 



" In my memoir on the equilibrium and motion of solid and 

 fluid bodies, I have deduced the function from the supposition 

 that the natural state of the body is one of free equilibrium. 



