132 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



the two dots are seen as one; but if far enough apart to 

 fall on two cones, they are seen as two. Taking the 

 foveal cones as one seventy-five-hundredth of an inch 

 (one three-hundredth millimetre) in diameter, and the 

 nodal point as six tenths of an inch (fifteen millimetres) 

 from the fovea, and the point of sight as ten inches, the 

 minimum visibile ought to be about one four-hundred-and- 

 fiftieth of an inch (one eighteenth millimetre). This is 

 about the fact for good eyes. If the point of sight be 

 six inches, as it may be in young normal eyes, the mini- 

 mum will be one seven-hundred-and-fiftieth of an inch 

 (one thirtieth millimetre). 



Comparison with Touch. — The only sense with 

 which we can make comparison in this regard is touch, 

 because these two are the only senses that take cogni- 

 zance of dimension. There is also a minimum tactile — 

 i. e., the smallest distance between two tactile impressions 

 in which they can be felt as two. This varies greatly in 

 different parts of the body. 



Experiment. — Take a pair of dividers, arm the points 

 with small shot or bits of cork, so as not to prick the 

 patient. Now try, on a blindfolded person, the distance 

 of separation between the points when these are felt as 

 two. It will be found that on the middle of the back 

 the separation must be two to three inches; on the out- 

 side of the forearm or back of the hand, about one half 

 or three quarters of an inch; on the finger tips, about 

 one twelfth of an inch; and on the tip of the tongue, 

 about one twenty-fifth of an inch, or one millimetre. In 

 the retina it is one seventy-five-hundredth of an inch. 



3. BLIND SPOT. 



We have already seen (page 118) that there is another 

 spot where all the layers of the retina are not present — 

 viz., just where the optic nerve enters the eye. As the 



