SENSE ORGANS. ! 3 3 



optic nerve consists of fibers and as it spreads these as 

 an innermost layer, all other layers must be absent here. 

 Now as the bacillary layer is the sensitive layer, it fol- 

 lows that the spot must be blind. 



Experimental Proof of a Blind Spot. — Experi- 

 ment i. — Make the spots on a sheet of paper, a few 

 inches apart, thus : 



Shut the left eye and look with the right steadily at 

 the left figure, a, while the paper or page is slowly 

 brought nearer the face. At a certain distance — about 

 nine inches for the above figures — the right-hand figure, 

 b, will disappear, but on continuing the approach it 

 again reappears. The image of b has traveled across 

 the blind spot and come out on the other side. 



Experiment 2. — Standing up, put a small coin on the 

 table and a finger of the left hand beside it. Now, shut- 

 ting the left eye and looking at the finger with the right 

 eye, move the finger slowly to the left and follow it with 

 the eye. At a certain point the coin disappears from view, 

 but reappears on continuing the movement of the finger. 



Experiment 3. — Look at a bright star with one eye, 

 say the right. Now move the point of sight slowly and 

 steadily to the left, horizontally. At a certain point the 

 star will disappear, only, however, to reappear on con- 

 tinuing the movement of the point of sight in the 

 same direction. In this manner almost any object, if 

 not too large, may be made to disappear from view. A 

 man sitting at a distance of. say, one hundred feet may 

 be made to disappear. 



Diagrammatic Illustration.— The condition under 

 which the disappearance occurs is represented in the 

 diagram (Fig. 88). The eyes, R and Z, are supposed to 



