446 THE OUTGO OF ENERGY 



The thickness of the lens, accommodated for 

 distant objects, is 3.6 mm. Mark this point. 

 Here the posterior surface of the lens intersects 

 the optical axis. The radius of the posterior sur- 

 face is 6 mm. Find therefore a point on the 

 optical axis 6 mm. in front of the posterior sur- 

 face of the lens. With this point as a centre 

 describe with a radius of 6 mm. the segment of 

 the arc that shall represent the posterior surface 

 of the lens. Mark upon this drawing the cardi- 

 nal points of the lens, as follows. 



Optical Centre. In the cornea, a simple spher- 

 ical surface, rays directed to the centre of curva- 

 ture, k, were found to pass through the refracting 

 surface unchanged in direction. In thin convex 

 lenses having a long focal distance it is generally 

 assumed that any ray passing through a point 

 within the lens termed the optical centre, o, is not 

 refracted. In thick lenses, on the contrary, every 

 ray excepting that coinciding with the principal 

 axis is refracted (see Nodal Points). 



The optical centre is situated in the prin- 

 cipal axis within the lens. In a lens bounded on 

 both sides by media of equal refracting power, 

 for example, the crystalline lens bounded by the 

 aqueous and vitreous humors, the optical centre 

 is found by dividing the axis of the lens, i. e., 

 the distance between the refracting surfaces on 



