THE BLIND SPOT. 



always opposite R that is, by keeping the gaze 

 on R. 



Consequently Li, L2, and L3 ought to be 

 seen by three different parts of the retina as 

 the sheet carrying the two letters is brought 

 nearer. 



But the middle one of these points of the 

 retina, opposite L2, and marked by O N, is the 

 point at which the optic nerve enters the eye, 

 before it is divided into branches terminating 

 in the sensitive ends all over the rest of the 

 retina which receive the impressions of light. 



The nerve itself, where it enters the eye 

 under the name of the optic disc, has none of 

 these sensitive nerve-endings in or over it. 



Now, the places to hear telephonic messages 

 are not at the entrance of the wires into the 

 exchange room, but at the instrumental ter- 

 mination provided for the purpose. In the 

 same way there is no perception of light over 

 the optic disc. It is a blind spot. 



And the arrangement for approximating 

 and removing R and L on a sheet of paper is 

 contrived for the purpose of ensuring an at- 

 tempt to see with this blind spot in such a way 

 that the failure to do so shall be observable. 



2. The Sane and Sober seeing Double. 



Sit where you can comfortably see a door- 

 knob, clock face, or some other well-defined 



219 



