540 THE HUMAN BODY. 



(.00016 inch) apart. The distance between the retinal images 

 of two points is determined 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. 160) are luminous points, the image of a wiU b& 



formed at a' on the prolongation of the line a n joining a 

 with the node, n. Similarly 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 d, 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 corre- 

 sponds 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 unstimu- 

 lated 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 prac- 

 tice raise our acuteness of visual discrimination; and in the 

 yellow spot which we constantly use all our lives in looking 

 at things which we want to see distinctly, we have educated 

 the visual apparatus up to about its highest power. Else- 

 where on the retina our discriminating power is much less 

 and diminishes as the distance from the yellow spot increases. 

 This is partly due, no doubt, to a less sensibility of those reti- 

 nal regions, such as, by other facts, is proved to exist, but in 

 part, no doubt is also due to a want of practice. The more 



