CHAPTER XXXII. 



THE EYE AS A SENSORY APPARATUS. 



The Excitation of the Visual Apparatus. The excita- 

 ble visual apparatus for each eye consists of the retina, the 

 optic nerve, and the brain-centres 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 stimulations of it to that cause, unless 

 we have special reason to know the contrary. As already 

 pointed out (p. 468) pressure on the eyeball causes a lumi- 

 nous 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 sensa- 

 tion 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. 135) lie near together in 

 the lens. If we v r ant 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 f Fig. 136), 



