482 THE OUTGO OF ENERGY 



ing from the near point will be united by the two 

 lenses in the same focal plane in which the rays 

 diverging from the far point were united by the 

 first lens. The second lens has " accommodated " 

 the optical system to the distance R P. 



In this experiment the power of the + 2 D lens 

 represents the distance R JP, or range of accom- 

 modation. The power of a lens is inversely pro- 

 portional to its focal distance A. Consequently, 



the range of accommodation l = 1 : A or -j. Then 



!_ ^__ 1_ 

 A~ P~ R 



In the eye the auxiliary lens necessary for 

 focussing the rays diverging from the near point 

 is provided by an increase in the convexity 'of the 

 crystalline lens. The difference in refractive 

 power of the two lenses (the crystalline in its 

 least convex form and the crystalline in its most 

 convex form) is the measure of the range of 

 accommodation of the eye. If the lens remain in 

 its least convex form, an auxiliary lens must be 

 placed before the cornea in order to bring rays 

 diverging from an object at the near point to a 

 focus on the retina. The strength of this auxil- 

 iary lens becomes then the practical measure of 



1 When 1 = 1 metre. 



