CHAPTER XVI 

 THE EYE AS A SENSORY APPARATUS 



The Excitation of the Visual Apparatus. The excitable visual 

 apparatus for each eye consists of the retina, the optic nerve, and 

 the brain-centers connected with the latter; however stimulated, 

 if intact, it causes visual sensations. In the great majority of 

 cases its excitant is objective light, and so we refer all stimula- 

 tions of it to that cause, unless we have special reason to know 

 the contrary. As already pointed out pressure on the eyeball 

 causes a luminous sensation (phosphene), which suggests itself 

 to us as dependent on a luminous body situated in space where 

 such an, object must be in order to excite the same part of the 

 retina. Since all rays of light penetrating the eye, except in the 

 line of its long axis, cross that axis, if we press the outer side of 

 the eyeball we get a visual sensation referred to a luminous body 

 on the nasal side; if we press below we see the luminous patch 

 above, and so on. 



Of course different rays entering the eye take different paths 

 through it, but on general optical principles, which cannot here be 

 detailed, we may trace all oblique rays through the organ by 

 assuming that they meet and leave the optic axis at what are 

 known as the nodal points of the system; these (kk', Fig. 92) lie 

 near together in the lens. If we want to find where rays of light 

 from A will meet the retina (the eye being properly accommodated 

 for seeing an object at that distance) we draw a line from A to k 

 (the first nodal point) and then another, parallel to the first, from 

 k' (the second nodal point) to the retina. The nodal points of 

 the eye lie so near together that for practical purposes we may 

 treat them as one (k, Fig. 93), placed near the back of the lens. 

 By manifold experience we have learnt that a luminous body 

 (A, Fig. 93) which we see, always lies on the prolongation of the 

 line joining the excited part of the retina, a, and the nodal point k. 

 Hence any excitation of that part of the retina makes us think 

 of a luminous body somewhere on the line a A, and, similarly, any 



231 



