THE SENSES. 529 



mencing only beyond its borders. If the light of a candle flame at 

 some distance be thrown upon the retina, it is perceived by the person 

 under observation, as well as its image by the observer. If the eye, 

 however, be turned in such a direction as to bring the image of the 

 flame upon the white circle of the papilla, this circle, and the nerve 

 fibres of which it is composed, are visibly illuminated to a certain 

 depth, owing to the translucency of their substance ; but the light is 

 no longer perceived by the person under examination. The moment 

 the image is allowed to pass beyond the limits of the white circle, its 

 light becomes perceptible. 



The Blind Spot. The region, accordingly, occupied by the entrance 

 of the optic nerve, from which the proper elements of the retina are 

 absent, is a blind spot, where luminous rays make no perceptible im- 

 pression. The diameter of this spot, according to the average measure- 

 ments by Listing, Hannover, and Helmholtz, is 1.65 millimetre, and it 

 covers in the field of vision a space of about 6 degrees. Notwithstand- 

 ing its existence, no dark point is usually observed in the field of vision, 

 for the following reasons : The blind spot is not situated in the visual 

 axis of the eye, but corresponds with the entrance of the optic nerve, 

 nearer the median line (Fig. 130). Consequently the image of an 

 object in the normal line of vision cannot fall upon this spot, but is 

 always outside of it, in the visual axis. Even an object perceived out- 

 side the direct line of sight can never reach the blind spot of both eyes 

 at once. If so placed that its image falls on the blind spot of one eye, 

 it will necessarily reach the retina of the other eye at a different point, 

 and will thus be perceived. But if one eye alone be employed, there 

 is always a small portion of the field of vision which is imperceptible. 

 This deficiency is not generally noticed, because it is in a part of the 

 field to which our attention is not directed, and where the distinction 

 of objects, under moderate illumination, is so imperfect, that the mo- 

 mentary absence of one is not regarded. It may, however, be 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 observable, even in indirect vision. 



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

 white cross in Fig. 137, the circular spot will also be visible, though 

 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, because its image has fallen upon the blind 

 spot ; while both within and beyond this distance it is again visible. 

 It may also be made to reappear by inclining the page laterally to the 

 right or left ; since this brings its image either above or below 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 



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