448 THE OUTGO OF ENERGY 



atecl between o and 7t 2 . Thus o and K^ are con- 

 jugate foci for the surface hi, and o and K 2 are 

 conjugate foci for the surface h z . KI and K 2 

 are virtual images of o; that is, if o were observed 

 through the surface hi the image would appear 

 to be KI, while if o were observed through A a 

 the image would appear to be K 2 . As the nodal 

 points are images of the same point o, they must 

 therefore be images of each other. 



The first nodal point, KI, which is the virtual 

 image of the optical centre, o, formed by rays 

 passing from o through the anterior, refracting 

 surface, hi, and which lies at the conjugate focus 

 of 0, is situated 2.126 mm. behind the anterior 

 surface of the lens (accommodated for distant 

 objects). The second nodal point, K z , the virtual 

 image of o formed by rays passing from o through 

 the posterior refracting surface, h 2 , is situated 

 1.276 mm. in front of the posterior surface of the 

 lens (accommodated for distant objects). The dis- 

 tance between the two nodal points is 0.198 mm. 



Principal Surfaces. Within a double convex 

 lens are two parallel planes, termed the principal 

 surfaces. They are perpendicular to the prin- 

 cipal axis and are so placed that the emerging 

 ray appears to come from a point in the second 

 principal surface that exactly corresponds to the 

 point in the first principal surface to which the 



