CHAP, in.] 



SIGHT. 



1153 



that the needle and indeed the whole field of vision seems dimmer. 

 When, however, the image is double on account of the eye 

 being accommodated for a distance greater than that of the 

 needle, blocking the left-hand hole causes a disappearance of the 

 right-hand or opposite image, and blocking the right-hand hole 

 causes the left-hand image to disappear. When the eye is ac- 

 commodated for a distance nearer than that of the needle, blocking 

 either hole causes the image on the same side to vanish. The 

 following diagram will explain how these results are brought about. 



Ttt 



\ 



FIG. 138. DIAGRAM OF SCHEINER'S EXPERIMENT. 



Let a (Fig. 138) be a luminous point in the needle, and ae, af 

 the extreme right-hand and left-hand rays of the pencil of rays 

 proceeding from the luminous point, and passing respectively 

 through the right-hand e, and left-hand /, holes in the card. 

 (The figure is supposed to be a horizontal section of the eye, 

 and a forms part of a transverse section of the vertically placed 

 needle.) When the eye is accommodated for a, the rays e and 

 / meet together in the point c, the retina occupying the posi- 

 tion of the plane nn ; the luminous point appears as one point, 



