264 Mr. A. S. Percival : Method of 



Refraction at a Spherical Surface. 



Fig. 4 represents a plane wave-front incident on a plano- 

 convex lens of radius 5 and thickness 3. As refraction only 

 occurs at the spherical surface where the light encounters 



the rare medium, - must replace /ju in the given equations. 



The critical angle when /* = 1-523 is 41° 2' 27*6", so this is 



Fig, 4. 



Refraction 



Plane Wave-front. r=5, 



/* = 1-523. 





0. 



, i 

 0. j n. 



y. /p. 



p. 



(°- 





 -27° 39' 50" 

 -41° 2' 28" 





 -45° 

 -90° 







3-5355 







-14-5602 -9-5602 



_ 7-4308 -3-8953 









 -3-5355 

 -5 





 24-5613 

 



the maximum value of <£, and the three values for the three 

 angles </>' (0, —45°, —90°) given in the table are sufficient 

 to trace the caustic. It will be noticed that at F" and at L 

 p = 0, F ;/ being a cusp and L being a point d' arret. 



Least Circle of Aberration. 



The extreme ray LTKL' cuts the axis at T and the caustic 

 at K ; D marks the position of the Least Circle and DK or 



