as applied to Micrometric Observations, 

 Fig. 6. 



453 



Hence, remembering that 



it follows that 



32A 2 M 2 



I 



sin 



■dx=7T, 



I 



2jrk 



(q+v) 



b\ 



2irk ( 



{ cos 2 -jr-(q + v)dv. 



Now if the angular dimensions of the source of light be 

 large compared with \/k, the limits of integration with regard 

 to v may be made infinite. In this case the intensity I 



s _ 8AW( 



b\ J -ao 



9 9_ 



dv {sin 2 — (a-hk)(q + v) + sm <2 j^-(a — k) {q + v) 



b\ 



bX 



-< ism * — ( q + v) + 2sm* 1 ^(g + v)}^ (—±—) 



4A 2 A 



7T 



{(a + *) + (a-*)— 2a + 2*}| ~^-^J 



= SAVik = constant. 



This result shows us that if the dimensions of the source 

 exceed a certain limit, no diffraction-fringes exist at all, at least 

 near the centre of tbe image. Next let the angular. dimensions 



Phil. Mag. S. 5. Vol. 47. No. 2$$. May 1899. 2 I 



