480 THE OUTGO OF ENERGY 



normal eye can read the letters numbered VI 

 If the subject sees these letters clearly the acute- 

 ness of vision is normal, and R is infinite. If the 

 subject reads I at one metre, II at two metres, 

 but cannot read VI at six metres, bring the test 

 card towards the eye until a point is reached at 

 which the letters numbered VI are seen clearly. 

 This is the far point. 



Near Point. 1 . Look through the holes in 

 the card used for Scheiner's experiment at a 

 needle placed vertically about 30 cm. (twelve 

 inches) in front of one eye. Obtain a single 

 clear image of the needle. Bring the needle 

 nearer the eye. As the distance between the 

 needle and the eye becomes shorter, the rays pro- 

 ceeding from the needle become more divergent 

 and require a greater convexity of the lens to 

 bring them to a focus on the retina. A point 

 will be reached at which the divergence exceeds 

 the utmost converging power of the dioptric ap- 

 paratus and the images received through the two 

 holes in the card can no longer be made to co- 

 incide on the retina ; the needle will then appear 

 double. This is the near point of accommodation 

 (punctum proximum p). The distance from p 

 to the eye P. 



Determination of Near Point. Hold in front of 

 the eye a test card containing print so small that 



