102 PROPKRTIKS or OPTICAL GKATINQS. 



This is also the angle between the maximum of the X 

 line and the maximum of one of wave length X + <1 X which 

 is just resolved from it (see section 10), and this is given 

 by the expression for the dispersion obtained in the last 



paragraph. Equating these two values of d 0, '" •^~*-'' ^ 



e 



= _ and E = Ne where N is the number of apertures. There- 

 j-; 



fore, _ N/in-— ':' tl Now t a n/d X is almost always 

 <i X \ d X •/ 



less than .1 of m. Thus the resolving power is given very 



nearly by Nm, the same expression that holds for the ruled 



grating. 



