508 



THE HUMAN BODY. 



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 

 internal causes. The field of vision is not absolutely dark 

 but slightly luminous, with brighter fleeting patches trav- 

 ersing it. These are especially noticeable, for example, in 

 trying to see and grope one's way with the eyes open up a 

 perfectly dark staircase. Then the luminous patches attract 

 special attention because they are apt to be taken for the 

 signs of objective realities; they become very manifest when 

 any sudden jar of the Body, due for example to knocking 

 against something, occurs; and have no doubt given rise to 

 many ghost stories. These visual sensations felt in the 

 absence of all external stimulation of the eyes, may for con- 

 venience be spoken of as due to the idio-retinal light. 



The Excitation of the Visual Apparatus by Light. 

 Light only excites the retina when it reaches its nerve end 



FIG. 137. 



organs, the rods and cones. The proofs of this are several. 

 1. Light does not arouse visual sensations when it falls 

 directly on the fibres of the optic nerve. Where this nerve 

 enters (p. 490) is a retinal part possessing only nerve-fibres, 

 and this part is blind. Close the left eye and look steadily 

 with the right at the cross in Fig. 137, holding the book 



