Bauhaus-Universität Weimar

Volltext Experimental Psychology: A Manual of Laboratory Practice, Vol. II: Quantitative Experiments, part 1: Student's Manual (2 (1))

Titel:
Experimental Psychology: A Manual of Laboratory Practice, Vol. II: Quantitative Experiments, part 1: Student's Manual
Person:
Titchener, Edward B.
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit16066/25/
```§ 2. Mental Measurement xxi
the beginning. In weighing our package, we had the zero point
upon the scale bar ; the limiting point at which the sliding weight
was just counterbalanced ; and the mark that lay I oz. (or i lb.)
from the zero point. There are various devices—the introduc¬
tion of submultiples of the unit, the use of. the vernier—for in¬
creasing the accuracy of measurement ; there are other devices
for standardising the conditions (temperature, stress) under
which a measurement is made ; there are mathematical rules for
calculating the ‘ probable error ’ of a given measurement. These
are all refinements of the art of measuring. The essential thing
is that we have our three terms : the limiting points of the mag¬
nitude to be measured, and a point lying at unit distance from
the one or the other limiting point.
The third term, without which measurement is impossible,
need not, however, be expressed. Suppose that two black strokes
are made upon a sheet of paper, and that you are asked to say
how far the one is above the other, or to the right of the other.
You reply, without difficulty, “Two inches’’ or “Five centi¬
metres.” But that means that you have mentally introduced a
third term : the unit mark, inch or cm., with which you have be¬
come familiar in previous measurements. Without this, you
could only have said : “ The one mark is above the other ” or
“to the right of the other” ; you could not have answered the
question “how far.” How long is a given stretch of level road ?
Two hundred and fifty yards? A quarter of a mile ? Most
people have no mental unit for such a measurement. Either
they say “ It looks about as long as from so-and-so to so-and-so,”
—comparing it with a familiar distance ; or they make a rough
determination by pacing the distance itself. How deep is this
well ? Very few people can say, even if they can see the water.
So a stone is dropped in, and the seconds are counted until the
splash is heard. The pace or the familiar distance gives a third
term for the measurement of the road ; and we know that the
distance traversed by the stone in falling is the product of the
distance traversed in the first second (about 490 cm.—our third
term) into the square of the time. Where there is no such third
term, there is no measurement. This rule is universal.
§ 2. Mental Measurement.—There can be no question but that,
```

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