113 



ASTRONOMICAL AND NAUTICAL 

 COLLECTIONS. 



i. Elementary View of the Undulatory Theory 0/ Light. 

 By Mr. Fresnel. 



[Continued jfrom the last Number.] 



I SHALL not undertake to explain here in detail the reasons 

 and the calculations which lead to the general formulas that 

 I have employed to determine the position of the fringes and 

 the intensity of the inflected rays : but I think it right to 

 give at least a distinct idea of the principles on which this 

 theory rests, and particularly of the principle of interference^ 

 which explains the mutual action of the rays of light on each 

 other. The name of interference was given by Dr. Young 

 to the law which he discovered, and of which he has made so 

 many ingenious applications. 



This singular phenomenon, so difficult to be satisfactorily 

 explained in the system of emanation, is on the contrary so 

 natural a consequence of the theory of undulation, that it 

 might have been predicted from a general consideration of 

 the principles of that theory. Every body must have ob- 

 served, in throwing stones into a pond, that, when two groups 

 of waves cross each other on its surface, there are points at 

 which the water remains immoveable, when the two systems 

 are nearly of the same magnitude, while there are other places 

 in which the force of the waves is augmented by their con- 

 currence. The reason of this is easily understood. The 

 undulatory motion of the surface of the water consists of ver- 

 tical motions, which alternately raise and depress the particles 

 of the fluid. Now, in consequence of the intersection of the 

 waves, it happens, that at certain points of their meeting, 

 one of the two waves has an ascending motion belonging to 

 it, while the other tends at the same instant to depress the 

 surface of the liquid : consequently, when the two opposite 

 impulses are equal, it can neither be actuated by one nor the 

 other, but must remain at rest. On the contrary, at the 

 points in which the motions agree in their direction, and con- 

 spire with each other, the liquid, urged in the same direction 



JULY—OCT. 1827. I 



