VISION. 209 



III. The distance from the second principal focus 

 to the principal point equals the product of the 

 distance from that focus to nodal point multi- 

 plied by the index of refraction of the second 

 medium (p = #'n). 



Note in addition the following facts regarding 

 the effect of such a dioptric system upon light. 



1st. The ray rs, meeting the spherical surface 

 perpendicularly, will not be refracted at s, but 

 will pass on through the nodal point. 



2d. The ray r's', parallel to the principal axis in 

 the first medium is refracted at the spherical sur- 

 face and cuts the principal axis at P, it passes 

 through the second principal focus. 



3d. The ray r"s", cutting the principal axis at f 

 in the first medium (M), is refracted at s" and 

 traverses the second medium parallel to the prin- 

 cipal axis. 



