THE RETINA 835 



and contains no other retinal element, it is insensitive to light. Nerve 

 fibers as such cannot be activated by the ethereal impacts of light. 

 The presence of the blind spot may be demonstrated in several 

 ways. Donders, 1 for example, projected the rays of a small flame 

 alternately upon the entrance of the optic nerve and upon the general 

 expanse of the retina. The individual received no sensation of light, 

 when the image was localized upon the former area. Another way 

 is this: If the left eye is closed, while the right eye gazes steadily at 

 the crossed lines of Fig. 439, the white circle situated about 8 cm. 

 to the right of this mark, becomes invisible as soon as the figure is 

 held at a distance of about 25 cm. from the eye. In other words, the 

 disappearance of this circle can only take place if the figure is placed 

 at a distance about 3 times greater than that between the cross and the 

 circle. If the latter are separated more widely, the figure must be 

 moved farther away from the eye and vice versa. Furthermore, if the 

 opposite eye is employed, the figure must be reversed, because the optic 

 nerve leaves the eyeball on the nasal side of a horizontal line drawn 

 through the anterior and posterior poles of the eye. 



FIG. 439. DIAGRAM TO DEMONSTRATE PRESENCE OF BLIND SPOT IN THE VISUAL FIELD. 

 Fix the cross with the right eye; bring figure closer to eye until the white dot dis- 

 appears. (Helmholtz.) 



Obviously, therefore, the visual field of each eye must possess an 

 indifferent area corresponding to the projection of the optic disc into 

 space. But, this projection does not give rise to a dark patch in space, 

 nor to a similar impression in consciousness but appears merely as an 

 area devoid of stimulation, i.e., as a small hole in the visual field with- 

 out any apparent details. Under ordinary conditions, this defect is 

 not apparent, because, when gazing at an object, we always deviate 

 the eye in such a way that the rays of light emitted by it fall upon the 

 most sensitive part of the retina, which is the yellow spot. Conse- 

 quently, the blind spot must occupy at this time a place in our indirect 

 field of vision. In binocular vision, the conditions are even more 

 favorable, because while an object or part of it, may fall upon the 

 blind spot of one eye, it cannot also do this in the opposite eye. Hence, 

 the blank in the field of vision of one eye is always filled in by the 

 other eye. A similar compensation is effected in the psychic center for 

 vision. 



The size and shape of the blind spot may be mapped out as follows : 

 Close the left eye and fix the right eye upon a mark upon a sheet of 



1 Onderzock., Physiol. Labor., Utrecht, vi, 1852, 134. 



