» THE RETINA 



837 



certain phenomena connected with direct and indirect vision, visual 

 acuity and the visual sensations produced by shadows. 



Direct and Indirect Vision. — Under ordinary conditions, the fixa- 

 tion of an object is accomphshed by turning our eyes in such a direc- 

 tion that its central area is brought to a focus upon the fovea centralis 

 of the yellow spot. The line uniting these two points constitutes the 

 line of most distinct vision, and is known as the visual axis of the eye. 

 Thus, in mapping out the details of a certain visual field, we invariably 

 direct our eyes first to one object or a part thereof, and then to another. 

 Only that object gives a perfectly clear impression at any one time 

 which projects its ray of light along the visual line directly into the 



vifl.e 



m.l.i 



Fig. 441. — Diagram of a Section Through the Fovea Centralis. (The outlines of 

 this figure have been traced from a photograph.) Magnified 350 diameters. (From 

 a preparation by C. H. Golding-Bird.) 



2, Ganglionic layer; 4, inner nuclear; 6, outer nuclear layer, the cone-fibers forming 

 the so-called external fibrous layer; 7, cones; m.l.e, membrana limitans externa; m.l.i. ^ 

 membrana limitans interna. 



fovea. Meanwhile, the other objects of this particular field appear 

 less distinct, because the rays emitted by them, form too great an 

 angle with the visual line and fall, therefore, upon the outlying regions 

 of the retina, where vision is less acute. In reading we invariably 

 fix one word after another. Quite similarly, it will be noticed that if 

 we gaze at a single word upon a page, the other words remain indistinct 

 and the more so, the greater the distance between them and the one 

 brought to a precise focus upon the fovea. The imaginary lines which 

 connect the outlying luminous points of an object with the more periph- 

 eral elements of the retina, constitute secondary visual axes. It will 



