458 THE OUTGO OF ENERGY 



at the principal focus. Produce izjz until it meets 

 iij\. The point of intersection will be the image 

 of the point i. 



.EEDUCED EYE 



The distance of less than one fourth millimetre 

 which separates one principal point from the 

 other is so small that it may be neglected with- 

 out any error of practical importance. Thus the 

 two principal points may be combined in one point 

 lying 2.34 mm. behind the anterior surface of the 

 cornea of the normal. 1 Similarly the two nodal 

 points may be combined in one point lying 

 0.48 mm. in front of the posterior surface of the 

 lens, or about 16 mm. in front of the retina. 

 The nodal point k of the cornea (System A) is 

 about 14 mm. in front of the retina. The nodal 

 point of the lens and that of the cornea are com- 

 bined in the reduced eye in a nodal point situated 

 15 mm. from the retina. The lens may therefore 

 be omitted. Indeed, the cornea is normally the 

 principal refracting surface ; its focal distance is 

 31.095 mm., while that of the crystalline lens is 

 50.617 mm. ; if the lens were not present, parallel 

 rays entering the eye would be focussed by the 



1 LISTING : Wagner's Haiidworterbuch der Physiologie, 1853, 

 iv., p. 495. 



