CHAPTER XXXIII. 



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-centres connected with the latter; how- 

 ever stimulated, if intact, it causes visual sensations. In the 

 great majority of cases its excitant is objective light, and sx> 

 we refer all stimulations 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 (Jck* ', Fig. 154) 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 Jc (the first 

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

 (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. 155), placed near the back of 

 the lens. By manifold experience we have learnt that a 

 luminous body (A, Fig. 155) which we see, always lies on the 

 prolongation of the line joining the excited part of the retina, 



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