768 EXPERIMENTS ON THE RETINA. 



cornea is bathed therewith. As the refractive index of water is almost the same as the refrac- 

 tive index of the media of the eye, the rays of light pass into the eye in a straight direction 

 without beiug refracted. Hence, objects in the anterior chamber can be seen directly, as if 

 they were not within the eye at all. Another advantage is that the objects can be brought 

 nearer to the eye of the observer. The rays of light emerging from the point (a) of the fundus, 

 if the eve were surrouuded by air, would leave the eye as the parallel lines, b, c, b, c. Under 

 water, these rays, a, b, continue in the direction a, b, as far as b, d, where they emerge from 

 the water, and are bent from the perpendicular to d, e, d, c. The eye of the observer, looking 

 in the direction c, d, sees the point, a, nearer, viz., in the direction c, d, a', lying at a. 



395. ACTIVITY OF THE RETINA IN VISION. I. Blind Spot The 

 rods and cones alone are the parts of the retina sensitive to light, they alone are 

 excited by the vibrations of the ether. This is confirmed by Mariotte's experiment 

 (1688), which proves that the entrance of the optic nerve, where rods and cones 

 are absent, is devoid of visual sensibility. Hence it is spoken of as the " blind 



[Mariotte's Experiment Make two marks, about 3 inches apart, upon paper 

 (fig. 556). Look at the cross with the right eye, keeping the left eye closed, and 

 hold the paper about a foot from the eye, when both the cross and the circle will 



+ 



Fig. 556. 



be seen. Gradually approximate the paper to the eye, keeping the open eye 

 steadily fixed on the cross ; at a certain moment the circle will disappear, and on 

 bringing the paper nearer to the eye it will reappear. The moment when the circle 

 disappears is when its image falls upon the entrance of the optic nerve.] 



Position and Size. The entrance of the optic nerve lies about 3*5 mm. internal to the 

 visual axis of the eyeball, in the retina. Its diameter is 1 '8 mm. The apparent diameter of 

 the blind spot in the field of vision is in a horizontal direction 6 56' this lies 12 35' to 

 18 55' horizontally from the fixed point. Eleven full moons placed side by side would dis- 

 appear on the surface, and so would a human face at a distance of over 2 metres. 



Proofs. The following facts prove that the entrance of the optic nerve is insensible to 

 light: (1) Donders projected, by means of a mirror, the small image of a flame upon the 

 entrance of the optic nerve of another person, and the person had no sensation of light. But 

 a sensation of light was experienced, when the image of the flame was projected upon the 

 neighbouring parts of the retina. (2) On combining with Mariotte's experiment the experiment 

 which causes entoptical phenomena at the entrance of the optic nerve, this coincides with the 

 blind spot ( 393, 6 and 7). 



Form of Blind Spot. In order to determine the form and apparent size of the blind spot in 

 one's own eye, fix the head at about 25 centimetres from a surface of white paper ; select a 

 small point on the latter and keep the eye directed towards it; then, starting from the 

 position of the blind spot, move a white feather in all directions over the paper ; whenever the 

 tip of the feather becomes visible, make a mark at this spot. The blind spot may be mapped 

 out in this way. It has an irregular, elliptical form from which processes proceed, clue to the 

 equally non-sensitive origins of the large blood-vessels of the retina (Hueck). (Mariotte 

 concluded from his experiment that the choroid, which is perforated by the optic nerve, is the 

 membrane sensitive to light, as the nerves are nowhere absent from the retina.) 



The blind spot causes no appreciable gap in the field of vision. As this area is not excited 

 bv light, a black spot cannot appear in the field of vision, for the sensation of black implies 

 the presence of retinal elements, which, however, are absent from the blind spot. The 

 circumstance, however, that in spite of the existence of^an inexcitable spot during vision, no 

 part of the field of vision appears to be unoccupied,' is due to a psychical action. The 

 unoccupied area of the field of vision, corresponding to the blind spot, is filled in according to 

 probability by a psychical process (E. H. Weber). Hence, when a white point disappears 

 from a black surface, the wnole surface appears to us black ; a white surface, from which a 

 black point falls on the blind spot, appears quite white ; a page of print, grey throughout, &c. 

 According to the probabilities, certain parts are supplied parts of a circle, the middle parts of 

 a long line, the central part of a cross. Such images, however, as cannot be constructed 



