THE EYE AS A SENSORY APPARATUS. 



531 



a, and the nodal point Jc. 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 excitation of b, of a body 



B 



FIG. 154. Diagram illustrating the points at which incident rays meet the retina. 

 xx, optic axi ; fc, first nodal point; fc', second nodal point; 6, point where the im- 

 age of B would be formed, were the eye properly accommodated for it ; a, the 

 retinal point where the image of A would be formed. 



on the line b B or its prolongation. It is only other conflict- 

 ing experiences, as that with the eyes closed external bodies 

 do not excite visual sensations, and the constant connection 







FIG. 155.- Diagrammatic section through the eyeball, xx, optic axis ; k, nodal 

 point. 



of the pressure felt on the eyelid with the visual sensation, 

 that enable us when we press the eyeball to conclude that, in 

 spite of what we seem to see, the luminous sensation is not 

 due to objective light from outside the eye. 



The Idio-Retinal Light. The eyelids are not by any 

 means perfectly opaque ; in ordinary daylight they still allow 

 a considerable quantity of light to penetrate the eye, as any 

 one may observe by passing his hand in front of the closed 

 eyes. But even in a dark room with the eyes completely 

 covered up so that no objective light can enter them, there is 

 still experienced a small amount of visual sensation due to 



