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 
