LOCALIZING POWER OF RETINA. 517 



are not more than .004 mm. (.00016 inch) apart. The 

 distance between the retinal images of two points is deter- 

 mined by the "visual angle" under which they are seen; 

 this angle is that included between lines drawn from them 

 to the nodal point of the eye. If a and b (Fig. 141) are 



FIG. 141. 



two points, the image of a will be formed at a' on the pro- 

 longation of the line a n joining a with the node, n. Sim- 

 ilarly the image of b will be formed at b'. If a and b still 

 remaining the same distance apart, be moved nearer the 

 eye to c and c?,then the visual angle under which they are seen 

 will be greater and their retinal images will be farther apart, 

 at c' and d '. If a and b are the highest and lowest parts 

 of an object,, the distance between their retinal images will 

 then depend, clearly, not only on the size of the object, but 

 on its distance from the eye; to know the discriminating 

 power of the retina we must therefore measure the visual 

 angle in each case. In the fovea centralis two objects seen 

 under a visual angle of 50 to 70 seconds can be distinguished 

 from one another: this gives for the distance between the 

 retinal images that above mentioned, and corresponds 

 pretty accurately to the diameter of a cone in that part of 

 the retina. We may conclude, therefore, that when two 

 images fall on the same cone or on two contiguous cones 

 they are not discriminated; but that if one or more un- 

 stimulated cones intervene between the stimulated, the 

 points may be perceived as distinct. The diameter of a 

 rod or cone, in fact, marks the anatomical limit up to which 

 we can by practice raise our acuteness of visual discrimina- 

 tion; and in the yellow spot which we constantly use all 

 our lives in looking at things which we want to see dis- 

 tinctly, we have educated the visual apparatus up to about 



