330 Mr KELLAND, ON THE TRANSMISSION OF IJGHT 



it could not produce: the former requires that the force should act 

 out of the plane of the wave, the latter rejects the part which does; 

 and these will right each other if we can shew (as I trust I have 

 done in the sequel) that the actual vibratory force is in the front of 

 the wave. I could have desired that my investigations should have 

 assumed a more inviting form, but I have not the means at present 

 of throwing tliem into a shape other than that under which they 

 appear. 



The first step I have taken is to prove the transversality of the 

 vibration, and thus having established a direction in which vibrations 

 do take place, I suppose that the forces put in play by a displace- 

 ment may be determined (as far as their action in the direction of 

 that displacement alone is concerned) in the same manner as Fresnel 

 does. The modification then Avhich I propose, consists in restricting 

 the theorem of Fresnel, and reducing it to the following : 



" That the whole vibratory force put in play by a displacement in 

 a direction ichich admits of a vibration, is the sum of the resolved 

 parts along that direction of the vibratory forces due to the resolved 

 parts of the displacement along the axes of elasticity." With a 

 direction normal to those of vibration I liave nothing to do, except 

 to prove that the force in that direction is not part of the vibratory 

 force. 



