IN FOCAL PLANE OF TELESCOPE. 



337 



§ 6. Intensity at any Point of the Focal Plane 

 due to a Circular Source of Light. 



Let us next find the intensity of light at any point in the 

 focal plane of the telescope, where m;iy lie either external or internal 

 to the image of the circular source. Put angle BCP=2<p, angle 

 BOP=d, and AO=d, AOB being the diameter passing through the 

 given point ; 



Q 



Fig. 2. 



Then 



OP 2 = o- = (a — of + of + 2 a (« — o)cos 2 <p 



= (2a— of — 4a (a — o)siri 2 <p , 



Aa(a— d) 



f) t = (2a— o) V 1— Äv «nV-.i where Av = 2q _ » 3 



Thus 



when lies within the circle ; similarly 



