REFRACTION IN THE EYE 447 



the principal a*xis, into two parts proportionate 

 to the radii of the refracting surfaces. 

 Then 



10 + 6 : 3.6 = 6 : x 



x= 1.35 mm., the distance of o from the pos- 

 terior refracting surface. 

 Then 



3.6 1.35 = 2.25 mm., 



the distance of o from the anterior refracting 

 surface. 



Nodal Points. In thick lenses (such as the 

 crystalline) with short focal distance, all the rays 

 except that which coincides with the principal 

 axis are refracted at one or both of the spherical 

 surfaces. In order to determine the path of rays 

 passing through the lens it is necessary to find 

 the nodal points. These are two points so placed 

 that a ray directed to the first point appears on 

 leaving the lens to have come from the second 

 point, in a direction parallel to the entering ray. 

 All rays coming from the optical centre, o, to the 

 anterior refracting surface will after refraction 

 appear to have come from the first nodal point, 

 KI, situated within the lens on the principal axis, 

 between o and hi. Similarly, all rays from o to 

 the posterior refracting surface will appear to 

 have come from the second nodal point, K^, situ- 



