222 RECEXT PROGRESS OF THE THEORY OF VISION. 



Now the gap caused by the presence of the optic nerve 

 is no slight one. It is about 6° in horizontal and 8° in 

 vertical dimension. Its inner border is about 12° hori- 

 zontally distant from the ' temporal ' or external side of 

 the centre of distinct vision. The way to recognise 

 this blind spot most readily is doubtless known to many 

 of my readers. Take a sheet of white paper and mark on 

 it a little cross ; then to the right of this, on the same 

 level, and about three inches off, draw a round black spot 



Fig. 32. 



half an inch in diameter. Now, holding the paper at 

 arm's length, shut the left eye, fix the right upon the 

 cross, and bring the paper gradually nearer. When it is 

 about eleven inches from the eye, the black spot will 

 suddenly disappear, and will again come into sight as the 

 paper is moved nearer. 



This blind spot is so large that it might prevent our 

 seeing eleven full moons if placed side by side, or a man's 

 face at a distance of only six or seven feet. Mariott^,^ who 

 liiscovered the phenomenon, amused Charles II. and his 



» Edme. Mariotte born in Burgundy, died at Paris, 1684, 



