SENSE OF SIGHT. 621 



noticeable, because it is located in a part of the field to which our at- 

 tention is not directed, and where the distinction of various objects, 

 under moderate illumination, is so imperfect, that the momentary ab- 

 sence of one of them is not regarded. It may, however, be readily made 

 apparent by using for the test a single strongly defined object, like a 

 white spot on a black ground, the presence or absence of which may be 

 noticed without difficulty, even in indirect vision. 



If the left eye be covered and the right eye directed steadily at the 

 white cross in figure 196, the circular spot will also be visible, though 



Fi>. 196. 



DIAGRAM, for observing the situation of the blind spot. (Helmholtz.) 



less distinctly, since it will be out of the direct line of sight. Let the 

 page be held vertically at the height of the eyes, and at a convenient 

 distance for seeing both objects in the above manner. If it be now 

 moved slowly backward and forward, a point will be found where the 

 circular spot disappears from sight, because its image has fallen upon 

 the blind spot ; while both within and beyond this distance it again be- 

 comes visible. It may also be made to reappear, even at the same dis- 

 tance, by inclining the page laterally to the right or left ; since this 

 brings the white circle either above or below the level of the blind spot. 



The experiment may be varied by fixing two cards, at the height of 

 the eyes, upon a dark wall, two feet apart from each other. If the left 

 eye be covered, and the right eye fixed upon the left-hand card, the 

 other one will disappear from view at a distance of about eight feet 

 from the wall. 



It is evident, furthermore, that the optic nerve fibres are not directly 

 sensitive to light, even outside the blind spot, and where they form 

 part of the retina. These fibres radiate from the point of entrance of 

 the optic nerve, forming a continuous sheet on the inner surface of the 

 retina ; some of them terminating at successive points in the retinal 

 membrane, others extending to its extreme border at the ora serrata. 

 A luminous ray striking the retina near the fundus of the eye must, 

 therefore, traverse a considerable number of nerve fibres, which are con- 

 nected at their peripheral extremities with different parts of the retina ; 

 and such a ray, coming from a single point, would necessarily cause the 

 sensation of multiplied luminous points or even of a more or less con- 



