POSITION OF RETINAL IMAGES. 



507 



placed near the back of the lens. By manifold experience 

 we have learnt that a luminous body (A Fig. 136) 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 



FIG. 135. Diagram illustrating the points at which incident rays in the eye 

 meet the retina, x x, optic axis; fc, first nodal point; fc', second nodal point; 

 6, point where the image of B would be formed, were the eye properly accom- 

 modated for it; a, the retinal point where the image of A would be formed. 



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 #, of a body on the line b B or its pro- 

 longation. It is only other conflicting experiences, as that 



FIG. 136. Diagrammatic section through the eyeball, 

 nodal point. 



xx, optic axis; fc, 



with the eyes closed external bodies do not excite visual 

 sensations, and the constant connection 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 



