222 Prof. Nihal Karan Sethi on Talbot's Bands and 

 When x x is very large 



01*1= «T J — =0. 

 The equations (4) and (6) reduce in this case respectively to 



I = 2(l + cosp')f 

 and i 



I = 2.^"-eosp'[4-3-l]|=^ 



so that when the slit is opened very wide we obtain dark 

 and bright bands in position P x o£ the plate, but uniform 

 illumination in position P 2 . 



The values of A and B in (6) and the corresponding 



o 



visibility — ■ for various widths of the slit are given below : — 



Table II. 



»i- A. B. Visibility = ? 



A 



j|r -28 -23 -84 



7T 



4 



8 



7T 



8 

 3tt 



•76 -50 -66 



1-47 -07 -05 



2-04 --33 --16 



2-43 -'07 -'03 



2-68 40 -04 



2-81 '017 -006 



4 



„. 2-83 -004 '001 



^5 2-93 '001 -000 



Ott 2-98 -001 -000 



Fie. 5 is a graphical representation of the visibility for 

 various slit widths. From this it is clear that the visibility 

 of the bands at first diminishes and becomes zero when the 



width of the slit corresponds to ^1= j nearly, i.e. when it 



