ON LIGHT. 331 



undulation or 100,000th of an inch, the length by which 

 the distances p a, P b, &:c., differ from each other, and 

 in that case it is very easy to show by geometry, that the 

 successive areas (a), (b), (c), Szc, are almost exactly 

 equal. Were these areas rigofviisly equal, and were 

 moreover the vibrations (propagated as they are from 

 them to p viorc and more obliquely with respect to the 

 general surface of the wave at their points of emanation) 

 all of equal intensity, it would follow therefore that the 

 otality of the movement propagated to p from (a) would 

 be precisely opposed and destroyed by that from (b), 

 tiiat from (c) by that from (d), and so on; so that an 

 ethereal molecule at p would in effect be agitated by no 

 preponderating movement, one way or another, and 

 there would be no illumination on the screen at P. In- 

 asmuch, however, as the vibrations diminish in mtensity 

 as they are propagated more obliquely, and as the areas 

 (a), (b), (c), (d), &;c,, are, though very nearly, yet not 

 rigorously equal, this mutual destruction in the case of 

 each consecutive pair is only partial, and the point P 

 will be agitated by the sum of all these outstanding 

 excesses (taken in pairs) from the centre outwards ; 

 which, though excessively small individually, in virtue of 

 their immense number make up a finite sum. And as 

 the same is true for each point of the screen (if spherical, 

 and therefore everywhere equidistant from o), the whole 

 of its surface will be equally illummated : if plane, very 

 nearly so, in all the region around p. 



(114.) A very singular consequence follows from this 

 reasoning, and one admirably calculated to test its 



