THE EYE AS A SENSORY APPARATUS 275 



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 unstimulated cones intervene between the stimu- 

 lated, 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 discrimination; 

 and in the fovea 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. Elsewhere on 

 the retina our discriminating power is much less and diminishes 

 as the distance from the fovea increases. 



While we can tell the stimulation of an upper part of the retina 

 from a lower, or a right region from a left, it must be borne in 

 mind that we have no direct knowledge of which is upper or lower 

 or right or left in the ocular image. All our visual sensations tell 

 us is that they are aroused at different points, and nothing at all 

 about the actual positions of these on the retina. There, is no 

 other eye behind the retina looking at it to see the inversion of the 

 image formed on it. Suppose I am looking at a pane in a second- 

 story window of a distant house: its image will then fall on the 

 fovea centralis; the line joining this with the pane is called the 

 visual axis. The image of the roof will be formed on a part of the 

 retina below the fovea, and that of the front door above it. I 

 distinguish that the images of all these fall on different parts of 

 the retina in certain relative positions, and have learnt, by the 

 experience of all my life, that when the image of anything arouses 

 the sensation due to excitation of part of the retina below the 

 fovea the object is above my visual axis, and vice versa; similarly 

 with right and left. Consequently I interpret the stimulation of 

 lower retinal regions as meaning high objects, and of right retinaJ 



