176 Mr. Barton on the Inflection of Light, 



the true origin of the undulations should be placed at the sun's 

 surface. I am sure, from the language of Professor Powell's 

 reply, that I did not explain myself on this head so fully and 

 explicitly as I ought. He says, " If the aperture be very small, 

 it is shown by theory that the new wave diverging from it, 

 which is produced by the sum of all the small waves belonging 

 to the original waves from RR/, &c, will be equally strong in 

 all directions ; and A in this case is the real centre and origin 

 from which the new undulations commence." 



In our sun, it is no doubt true, as observed by Professor 

 Powell, that the wave diverging from the point of intersection 

 " is produced by the sum of all the small waves belonging to 

 the original waves from RR' &c." It is no doubt true, on the 

 principles of the undulatory theory, that the particles of aether 

 composing the wave R'A, on arriving at the point A are agi- 

 tated by a multitude of impulses derived from other points 

 of the sun's surface ; the succeeding portion AY may there- 

 fore be considered as the joint result of all these impulses. But 

 inasmuch as the disturbing forces in question are infinite in 

 number, and infinitely various in force and direction, they 

 completely counteract each other's operation ; and the practi- 

 cal result is, that the wave emanating from the point W goes 

 forward to its ultimate destination at Y, precisely as if it had 

 not been crossed in its path by any other rays whatever. Such 

 at least was the doctrine of Huygens, the great author of the 

 undulatory theory. If the undulationists of the present day 

 have seen it necessary to abandon or modify the principles of 

 their master, it appears incumbent on them to assign some 

 reason for the change, and to show how it may be reconciled 



with the laws which are admitted to regulate the undulations 

 of elastic media. 



According to Young, the point Y will always be of double 

 brightness if the paths of the two rays AY, AKY differ in 

 length by an integral number of undulations. But it is evident 

 that the chances are infinite against the two rays RA, R'A, 



