OPTICS 263 



In the condition of strong accommodation the radius of the 

 anterior surface of the lens is 6 mm, of the posterior surface 

 5.5 mm ; the anterior surface is also slightly pressed forward, 

 the posterior surface remains in its position. For this condi- 

 tion also a reduced eye may be constructed, the radius of the 

 spherical surface of separation being 4.53 mm, its centre of 

 curvature lying 6.79 mm behind the vertex of the cornea. 

 In this condition sharp images of objects 120 mm distant 

 from the vertex of the cornea are formed upon the retina. 



Purkinje-Sanson images. The change in the curvature 

 of the lens shows itself by changes in the size and position 

 of the image reflected from the surfaces. If a candle is held 

 on one side of the eye, three reflected images may be seen 

 if the eye is looked at from the other side (Fig. 29, / r "). 

 The first image, formed by the cornea, is upright and clear; 

 the second, formed by the anterior surface of the lens, lies 

 near and a little behind the first; the third, formed by the 

 posterior surface of the lens, is small and inverted. If the 



F N 



FIG. 29. 



eye is accommodated for near vision, b and c become smaller 

 and /; approaches a (Fig. 29, N) ; this is accomplished by 

 the stronger curvature of the surfaces of the lens. 



Schemer's experiment. In a cardboard make two pin-holes 

 near each other. Place the holes near one eye and, the other eye 

 being closed, look in the direction of a near point. If now the 

 experimenter look past the ppint into the distance, the first point 

 is seen double ; if the near point is looked at, only one point is 

 si'L'n. Of the rays emanating from the near point two thin bundles 

 pass through the pin-holes into the eye, which, when the eye is 

 accommodated, unite to form one image point on the retina ; but 



