By similar manipulations the simple needle may be made to feel like
three or like four; now standing in a line, now in a triangle, and
again in the corners of a square. But, since there is but one needle,
what about the apparent distance _between_ these several points that
are clearly _felt_? This is the most curious thing of all, and from
the light it throws on the formation of our "ideas" both of number and
of space, is the most important.
To explain this: our notion of distance results out of "series" of
sensations, in the same way as our notions of number. To have any idea
of "distance" aroused between any two points of skin, the line of
nerves lying between those points must, some time during life, have
been previously stimulated in a line of succession, such as would
result from a pencil drawn along between them. A card edge would give
no idea of "distance" until such a series had some time been
previously experienced. The memory of the "series" _is_ the idea of
the distance.
Within small areas of the skin, so few "series" have been experienced
that no "distance memories" have been developed. Consequently
pin-point areas commonly awaken no notion of distance. For some
regions of the body these "limit areas" are larger than for others; at
some places are quite large. On the back, spaces three inches apart
may fail to give any idea of number or of distance. Every region has
such a limit distance.
_Now it is this limit distance, the smallest distance for which a
"series" memory has been developed for a given region, that always
shoves itself in, as the apparent distance between the several
fictitious points felt from the single needle in our experiment. On
the back the one needle feels like two set three inches apart; on the
forehead like two half an inch apart; on the tongue one-sixteenth of
an inch; and so on._
The upshot, then, of this matter is to show that our whole mind--our
notions of space, number, time, and all else--is but a bundle of
lawful habits, formed in relation with the things and occurrences
around us. Ordinarily we have right ideas, because on the whole our
mind has formed right habits. We have the right idea of an inch of
skin, because the proper idea of an "inch long" has become habitually
joined to each inch of skin, or in so far as this has been done. When
a wrong idea gets joined, then we have an illusion; that is, the
stretch of skin, or, as well, the pin-point of skin, seems a fraction
of an inch in length; or, again, like three inches.
"TIME REACTIONS:" METHODS OF MEASURING THE TIME REQUIRED FOR
PERFORMING VARIOUS MENTAL ACTS.
A sketch like this would be incomplete without a word about time
reactions--a subject that historically was almost the first in the
field, and has occupied more workers than any other. A generation ago
"as quick as thought" was our extreme limit of expression. It outran
"quicker than lightning." The great physiologist, Johannes Mueller,
wrote, in 1844:
"We shall probably never secure the means of ascertaining the
speed of nerve activities, because we lack the comparative
distances from which the speed of a movement, in this respect
analogous to light, could be calculated."
We now know that sensory processes travel along the nerves on an
average only about one hundred and ten feet per second, and often less
than twenty-six feet. While you are performing the commonest judgment,
electricity or light would have shot from continent to continent. The
time-measurement of different mental processes is now one of the chief
means which the psychologist uses for getting at mental laws. When
certain measures are once determined, he uses these as the chemist
does his familiar reagents, to dissolve the unfamiliar and more
complicated combinations.
The following table shows in decimals of a second about the average
length of time which our commonest judgments occupy: