CHAP, ii.] 



SIGHT. 



495 



for a, the rays e and / meet together in the point c, the retina 

 occupying the position of the plane nn; the luminous point appears 

 as one point, and the needle will appear as one needle. When the 

 eye is accommodated for a distance beyond a, the retina may be 

 considered to lie * no longer at nn, but nearer the lens, at mm for 

 example ; the rays ae will cut this plane at p, and the rays of at 

 q; hence the luminous point will no longer appear single, but will 



FIG. 67. DIAGRAM OF SCHEINER'S EXPERIMENT. 



be seen as two points, or rather as two systems of diffusion circles, 

 and the single needle will appear as two blurred needles. The 

 rays passing through the right-hand hole e, will cut the retina at 

 p, i.e. ou the right-hand side of the optic axis; but, as we shall see 

 in speaking of the judgments pertaining to vision, the image on 

 the right-hand side of the retina is referred by the mind to an 

 object on the left-hand side of the person ; hence the affection 

 of the retina at p, produced by the rays ae falling on it there, gives 



1 Of course, in the actual eye, as we shall see, accommodation is effected by a 

 change in the lens, and not by an alteration in the position of the retina ; but for 

 convenience sake, we may here suppose the retina to be moved. 



