862 



THE SENSE OF SIGHT 



1.454 mm. and that lines drawn from its two opposite poles through the 

 nodal point of the lens, subtend an angle of 60 degrees. Consequently, 

 any other two luminous points separated by a shorter distance than 



Fig. 461. — Diagram to Illustrate the Refraction in Emmetropia and Ametropia. 

 E, Emmetropic eye in which luminous point L is brought to a precise focus upon the 

 retina, L^; M, myopic eye in which L is focalized in front of the retina, L^; H, hyper- 

 metropic eye in which L is focalized in L^" behind" the retina. In M, the use of a concave 

 lens f orces L^ backward upon the retina, L^, correcting the myopia, whereas in H, the 

 use of a convex lens forces L^ forward upon the retina, L^. 



the one just given, are no longer able to produce distinct impressions. 

 At this distance, the retinal image measures 0.004 mm., which corre- 

 sponds to a visual angle of 60 seconds. If smaller than this, the two 



5ni. "^ — 1^- 



Fig. 462. — Dlvgram to Illustrate the Use of Snellen's Test Types. 



focal points fail to give separate impressions, because they fall on one 

 and the same cone. At a distance of 1 m., therefore, an object would 

 have to possess a dimension of one-fifth of 1.454 mm., or 0.2908 mm., 



