330 



' II. XAGAOKA: DIFFRACTION PHENOMENA 



effect of the zone of nearly the same breadth varies almost inversely 

 as its distance from the centre. The same reasoning will apply to a 

 circular sector. The following tables give the intensity at the centre 

 of luminous discs whose radii are equal to the roots of J" 1 (;r) = 0. 

 It will be found from the table that the effect of the first zone is 5 

 times greater than that of all the rest combined. Thus the illumi- 

 nation at the centre of a luminous circular disc of small radius will 

 not sensibly differ from that of infinite extent. We shall have 

 occasion to discuss the illumination of the disc whose radius is less 

 than X] in a subsequent section. 



