74 VISION. 



peated movement at a definite rate, is in a very 

 different position, and has exactly the same objec- 

 tive reality as space and time, whatever that 

 may be. 



In examining the characters of the channels of 

 communication between the mind and the world 

 afforded by the senses it will be convenient to con- 

 sider for a moment the mechanism of touch before 

 passing on to vision. 



Suppose that you put your finger in contact with 

 the point of a pin, and let us try to make out as 

 far as we can the mechanism by which you become 

 aware of that very simple matter. In your finger 

 there are numbers of threads, each branching at the 

 end into a brush of filaments so densely distributed 

 that you cannot place the pin point on any part of 

 the skin where it will not be microscopically close to 

 some of them. These threads, called nerves, run up 

 to the spinal marrow ; and when your finger touches 

 the pin it is most certain that a change takes place 

 in the end of a nerve and quickly travels along it, at 

 a rate of more than one hundred feet per second, till 

 it reaches the spinal marrow. And even then you do 

 not feel the pin-point; for if your spinal marrow were 

 divided at the top of the neck, as has happened to 

 some unhappy persons, we might pinch and burn 

 your hand, and, though it would wince and jerk so 



