in He-ply to Professor Powell. 1 77 



arriving at the point A in the same state: their conditions 

 must be as diversified as the lengths of the paths which they 

 have severally traversed in arriving from the sun. What 

 then is the nature of the force which operates upon them at 

 A, the point of intersection, to bring them, or one of them, 

 into a new condition ? Suppose, for instance, that the wave 

 forming the ray RA, on arriving at the point A, happens 

 to be at its maximum of advance, while the wave forming 

 the ray R'A, on arriving at the same point, happens to be 

 at its maximum of retrogression. By the theory of Huygens, 

 each will go forward in its course quite independently, as 

 if the other did not exist. How happens it, then, that on 

 arriving at K and K' they are found in one and the same 

 state ? One or both of them must have experienced an ac- 

 celeration, or a retardation, such as to make up together 

 the length of half an undulation. How has this been accom- 

 plished ? 



In the theory of Young, respect is had only to the two rays 

 RA, R'A, on whose interference the colour of the point Y is 

 supposed to depend. In the theory of Fresnel, the new wave 

 diverging from A is supposed, as Professor Powell truly says, 

 to be produced by the sum of all the small waves belonging 

 to the original waves from R R', &c. But this, though a more 

 accurate way of stating the question than Young's, gives in 

 practice very nearly the same results ; inasmuch as those parts 

 only of the wave entering the chamber which follow the di- 

 rection of the lines AY, AKY, do in fact constitute effectual 

 rays ("rayons efficaces"), as Fresnel has himself observed. 

 It is essential to his theory that every part of the hemispheri- 

 cal wave K'K should be found in one and the same condi- 

 tion, as it is to the theory of Young that the two individual 

 rays should be found in like condition at K' and K. The 

 simple question which I would ask of the undulationists is, 

 how has this equality of condition been brought about? 1 do 

 not see how it is possible, on their theory, to give an answer to 

 this question. 



If, however, we suppose light to consist of material par- 

 ticles, endued with a mutually repulsive force, and so consti- 

 tuting an elastic fluid of great tenuity, the question just pro- 

 posed may be solved without difficulty : for that such a fluid 

 in passing through a narrow aperture, when pressed against 

 an opposing surface, will be thrown into a series of undulatory 

 movements, may be shown, I think, either by reference to the 

 known laws of the motion of fluids, or by direct experiment. 

 Take a tube, an inch or more in diameter, closed at one end, 



Third Series. Vol. 3. No. 15. Sept. 1833. 2 A 



