516 VISION 



If the left eye be closed and the right be fixed steadily on the white cross 

 in Fig. 212 and the book be held at a distance of about 25 cm. from the eye, 

 the white circle will disappear entirely from view, so that the black field 

 appears uniform. There is, therefore, in the eye a spot which is not sensitive 

 to light, and which is called for this reason the blind spot. 



By measuring the apparent size of the blind spot, and its apparent distance 

 from the fixation point of the eye, it can be shown to correspond exactly with 

 the point of entrance of the optic nerve, where the mass of optic fibers, not 

 covered by the black pigment, spreads outward toward the transparent media 



FIG. 212. 



of the eye. The insensibility of the optic nerve fibers appears still more 

 directly, if by means of a small mirror the light of a small flame be thrown 

 into the eye so that it falls upon the point of entrance of the optic nerve. 

 The subject experiences no sensation of light (Danders). 



The blind spot is so large that at a distance of 1.7-2 m. it can contain the 

 image of a man's head. The reason why we do not ordinarily miss the object 

 in our field of vision which falls upon the blind spot is, that we unconsciously 

 fill the gap with something conformable to the rest of the field. Moreover the 

 distance of the blind spot from the center of exact vision is such that objects in 

 that quarter would be pretty indistinct if the spot were not blind. 



The light-perceiving layer of the retina, therefore, must lie behind the 

 nerve-fiber layer, or still more accurately behind the blood vessels of the retina, 

 as was first shown by the famous experiment of Purkinje. 



If a beam of light from a short-focus lens be concentrated on the con- 

 junctiva of one eye as far as possible from the cornea, and at the same time 

 the gaze of this eye be directed toward a uniformly colored dark background, 

 there appears at once in the field of vision a network of dark, branching 

 vessels. This network is nothing else than the shadow of the vessels of the 

 retina. 



Purkinje's figure, as this vascular tree is called, is rendered still more 

 plainly visible if the illuminating lens be moved to and fro; it can also be 

 perceived, if while the gaze is directed to a dark background a burning candle 

 be moved to and fro at one side and a little below the eye. 



From the fact that we can perceive the shadow of the retinal vessels in our 

 own eyes, it follows that the vessels themselves are in front of the light- 



