322 PROFESSOR KELLAND ON THE THEORETICAL INVESTIGATION OF 



and angular distance from a plane passing through the axis of the lens and the 



point M is 6, is 



Now 



a r d rd (9 sin 2 ~(v t-PM). 



P M = B -|-cos Q nearly ; and the vibration becomes 



-=r d r d 6 sin -j- ( v t B + -- cos 6 ) 



a r e /* 2 * /27T ro .\ 



jj/ j o rdrddsm^ -j-cos6\ =M 



-)-* 



then the intensity at the point M is M 2 + N 2 . 



o /2 

 Now M^^ 



27TOC /j 



where <p = ~YJ cos y - 



sin 



where /2r is an abbreviation for 1 . 2 . 3 . . . 2 r. 



Now f d 6 (cos 6) 2r+l =o .-. every term of the above expression is 



J 



zero; and consequently M=0. 



Again, N =4- / d 6 I ~ 5~~g sin + (OTTO ^) ( cos - !) 



Denote ^f- by m. 



XT a 2 /^ 2 T j a / sin (m cos 6) 1 cos (OT cos 6) 1 

 = ^ I mcos6 m cos 0) 2 J 



