472 THE OUTGO OF ENERGY 



which the diameter of the cone at its base 

 (4 mm.) is reduced at the retina. In the example 

 taken y = 1 mm. Then the proportion sought is 

 1 : 19 + 1 = -gV Thus the diameter of the dis- 

 persion circle is ^V of 4 mm. = | rnm. 



3. Calculate the size of the dispersion circle 

 produced by an object twelve metres from the 

 emmetropic eye. At this distance the dispersion 

 circles are so small as to cause no perceptible 

 lack of clearness in the image. 



Accommodation Line. Hold a needle two 

 inches from a printed page. Bring the eyes as 

 near the needle as is possible without causing the 

 image of the needle to blur. When the needle 

 is at this " near point " of accommodation it will 

 be seen clearly, but the printed words will be 

 indistinct. Draw back the eyes gradually. 



Soon a point will be reached at which both 

 needle and print will be seen distinctly. The 

 greatest distance between two objects on the 

 visual line at which the two may both be seen 

 clearly while the eye is accommodated for either, 

 is called the accommodation line. The length of 

 the accommodation line increases as the object is 

 removed from the eye. 



