450 THE OUTGO OF ENERGY 



division of the distance between the refracting 

 surfaces is the same for both s and o, and there- 

 fore s and o coincide. In System C, on the con- 

 trary, the first medium is the air, and the last 

 the vitreous humor. The principal focal dis- 

 tances are proportional to the coefficients of re- 

 fraction of the first and last media ; they are 

 no longer proportional to the radii of curvature ; 

 therefore s and o no longer coincide, and their 

 images, lying in the principal surfaces and at 

 the nodal points, respectively, no longer coincide, 

 but must be found separately. 



Principal Points. At the intersection of the 

 principal surfaces with the principal axis lie the 

 principal points, 1 H^ and ZT 2 . The second princi- 

 pal point is the image of the first. Eays which 

 in the first medium are directed to the first 

 principal point are directed to the second princi- 

 pal point in the last medium, i. e. } after the last 

 refraction. The anterior principal focal distance 

 is calculated from the first principal point, and 

 the posterior principal focal distance from the 

 second principal point. 



Principal Focal Distances. The posterior focal 

 distance of the lens (accommodated for distant 



1 The principal points, ff l and H z , coincide with the nodal 

 points, KI and JT 2 , when the first and last media of the optical 

 system have the same refractive power. 



