THE SENSES 813 



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 corresponding 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 



FIG. 313. MARIOTTE'S EXPERIMENT. 



be found in which the white disc disappears altogether. In this 

 position its image falls on the blind spot. (See Practical Exercises, 

 Figs. 332, 333.) 



Relation of the Rods and Cones to Vision. We have more 

 than once referred to the rods and cones as the sensitive 

 layer of the retina. It is now necessary to develop a little 

 more the evidence in favour of this statement. And at 

 the outset, since the sensitive layer has been shown to lie 

 behind the plane of the retinal bloodvessels, the only com- 

 petitors of the rods and cones are the external nuclear 

 layer and the pigmented epithelium. The nuclear layer 

 may be at once excluded, because in the fovea centralis, 

 where vision is most distinct, it becomes very thin and 

 inconspicuous. 



The layer of pigmented hexagonal cells, or at least their 

 pigment, cannot be essential to vision, for albino rats, 



