1004 



THE SENSES 



The Blind Spot The fibres of the optic nerve are insensible to 

 light ; light only stimulates them through their end-organs. This 

 can be proved by directing by means of an ophthalmoscope a beam 

 of light upon the optic disc, where the true retinal layers do not 

 exist The person experimented on has no sensation of light when 

 the beam falls entirely upon the disc ; when its direction is shifted 

 so that it impinges upon any other portion of the retina, a sensation 

 of light is at once experienced. The blind spot is not recognized 

 in ordinary vision, for (i) the two optic discs do not correspond. 



The left disc has its corre- 

 sponding points on a sensitive 

 part of the right retina, and 

 the right disc on a sensitive 

 part of the left retina ; and the 

 consequence is that in binoc- 

 ular vision the objects whose 

 images are formed on the cor- 

 responding points fill up the 

 blind spots. (2) The optic 

 disc does not lie in the line of 

 direct, and therefore distinct, 

 vision. The eye is constantly 

 moving so as to bring the 

 surrounding objects succes- 

 sively on the fovea centralis; 

 and the gap which the blind 

 spot makes in the visual field 

 of a single eye is thus more 

 easily neglected. In any case 

 we ought not to see it as a 

 dark spot, for darkness is only associated with the absence of 

 excitation in parts of the retina capable of being excited by light. 

 There is no more reason why the optic discs should appear dark 

 than there is for our having a sensation of darkness behind us when 

 we are looking straight in front. And since the experience of our 

 other senses the sense of touch, for example tells us that the 

 objects we look at do not in general have a gap in the position corre- 

 sponding to the part of the image that falls on the blind spot, we 

 see, so to speak, across the spot. 



By Mariotte's experiment, however, the existence of the blind spot 

 can not only be demonstrated, but its size determined and its boundaries 

 mapped out. Let the left eye be closed, and fix with the right the small 

 cross; then, if the eye be moved towards or away from the paper, keeping 

 the cross fixed all the time, a position will be found in which the white 

 disc disappears altogether. In this position its image falls on the 

 blind spot (Fig. 425). 



Fig. 424. Method of rendering the Blood- 

 vessels of the Retina visible by Oblique 

 Illumination through the Cornea. Light 

 from a candle at a illuminates a', and 

 rays proceeding from a' cast a shadow of 

 the bloodvessel, v, at a", which is referred 

 to a"'. When a is moved to b, the 

 shadow on the retina moves to b", and 

 the shadow in the visual field of the illu- 

 minated eye to b'". 



