IN FOCAL PLAXE OF TELESCOPE. 



Ul 



<p, <p is very nearly a right angle for points of the periphery included 

 within the circle AB. 



Let <p =-q— fi, where </> is a very small angle, then 



1 — A;- sin 2 <p 



> + » 



'</'' — It* %f- nearlv. 



Denoting the angle subtended by AB at the centre of the circulai- 

 image by 4 </'i, we obtain by the formula already given 



Expressing h' in terms of« and o, -we obtain 



V i 



8 4a{a — o) 

 Practically a is a large quantity and </\ a 

 very small angle, so that we may safely neglect 



-^- , and put 



^'i- = 



flV — <r 



4a(a — d) 

 and for points on the periphery AB 



Writing ç = ^/4a{a-ô)l< , 

 ç l = ^/4a(a — ô) (/• 1 = xî 2 — ô- nearly, we get 



and dp=—d<p= 



^/4a{a — o) 



Returning to formula (II) §1, we find 

 the intensity at due to the sector OAB 



where 2« is the angle subtended by the arc 

 AB at 0, 



Kg- 3. 



