532 THE HUMAN BODY. 



internal causes. The field of vision is not absolutely dark 

 but slightly luminous, with brighter fleeting patches travers- 

 ing it. These are especially noticeable, for example, in try- 

 ing to see and grope one's way with the eyes open up a per- 

 fectly 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 ab- 

 sense of all external stimulation of the eyes, may for conveni- 

 ence 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 

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



FIG. 156. 



1. Light does not arouse visual sensations tvhen it falls 

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

 enters there 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. 156, holding the book verti- 

 cally in front of the face, and moving it to and fro. It will 

 be found that at about 25 centimeters (10 inches) off the 

 white circle disappears ; but when the page is nearer or 

 farther, it is seen. During the experiment the gaze must be 

 kept fixed on the cross. There is thus in the field of vision a 

 Uind spot, and it is easy k> show by measurement that it lies 

 where the optic nerve enters. 



When the right eye is fixed on the cross, it is so directed 

 that rays from this fall on the yellow spot (y, Fig. 157). 

 The rays from the circle then cross the visual axis at the 

 nodal point, n, and meet the retina at o. If the distance of 

 the nodal point of the eye from the paper be/, and from 



