IN FOCAL PLANE OF TELESCOPE. 



333 



The correction is very small, if « be not very small ; for the sake 

 of convenience, I have calculated the following approximate values 

 for the zones 2, 3, 4, 5. 



§ 5- Intensity at the Rim of a circular Disc. 



The present problem is directly connected with the theory of 

 astronomical instruments, and has important bearing on the observa- 

 tion of celestial bodies. The simplest and perhaps the most important 

 calculation we can make is that of the case of a luminous circular 

 disc. We have already seen how the intensity at the centre of a 

 circular disc can be easily calculated ; we shall now proceed to the dis- 

 cussion of the intensity at its rim, and then obtain the result for the 

 general case when the point lies outside or inside the image of the 

 disc. 



Let the radius of the disc be a, then the distance of a point on 

 the periphery will be given by 



r = 2a cos 6 



The intensity at the rim r= is given by 



7T SaeosQ 



I= ^r/WL drdd 



7i J 'o r 



Integrating it first with respect to #, we can put the integral under 

 the form 



