158 



NUMERICAL APERTURE, 



B C meet it at C. From O draw O D perpendicular to B C, and 

 meeting it in D ; the line O D will represent the tangent to the 

 advancing waves. Now, when the wave advancing along B D has 

 reached C, the wave advancing along A O has entered a certain 

 distance into the second medium ; and this distance has the pro- 

 portionate length, to the length D C, that the speed of advance- 

 ment in the second medium has to that in the first : with the 

 centre O, and this multi|jle of the distance D C, describe, a semi- 

 circle, P H L. In Fig. 2, a, this distance is smaller thin D C, 



Fig. 2, a. 

 and in Fig. 2, l>, is greater ; (in f^ict, the two figures are drawn to 



Fig. 2, b. 



