848 



ACCOMMODATION. 



[BOOK iii- 



distance of the needle, the only effect produced by blocking up 

 one hole of the card is 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 dis- 

 appear. When the eye is accommodated 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. 



FIG. 143. DIAGRAM OF SCHEINER'S EXPERIMENT. 



Let a (Fig. 143) be a point in the needle, and ae, of the 

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

 proceeding from that 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 



