# Volltext: Elementary course in psychological measurements (4)

```Elementary course in psychological measurements. 139
The subject sees the color alternate with black, at first slowly, then
rapidly. Soon they appear to mix, but still retain a flickering appear¬
ance. As the speed continues to increase, the flickering becomes less
marked and finally diappears. At the moment when the illumination of
the field appears to be constant and steady the subject says “Now.”
The experimenter notes and records the position of the pointer at this
moment.
The speed of the motor is allowed to increase considerably beyond the
point just recorded ; the current is then turned off. The speed gradu¬
ally decreases and at some point the illuminated field begins to flicker.
At this point the subject says “Now,” and the experimenter notes and
records the position of the pointer.
Records of the first kind may be called “up-records,” those of the
other “down-records.” The up-records are placed in one column, the
down-records in another.
After a few preliminary experiments to obtain practice, ten records are
Computation.
Let m be the average of the up-records and n that of the down-rec¬
ords. The time of one revolution in seconds is thus 1 fm, or i/n, and
the time of half a revolution in ym or yn.
Let the suppositions be made : 1, that the black was equivalent to
absence of light ; 2, that there was no lag at the appearance of the red.
It follows that since the red seemed to be present all the time, whereas it
was present only half the time, the sensation must have persisted through
the time of y2 a revolution, i. e., 2m or 2n sec., without any percepti¬
ble diminution of intensity.
The suppositions are not strictly according to fact. The black
of the disc is not an absolute black, and there is a small lag at the be¬
ginning of the red sensation. For a bright red and a cloth black these
errors may be neglected, as in the present case.
Below the records the following statements are to be made (all figures
being in decimals to the 1/100 and the unit being seconds) : lag, up,
= ; lag, down, = ; lag, average, = .
Points to be noted.
i. The lag may be dependent on the intensity of the light and the
area of the field. 2. The lag is a psychical and not a physical affair.
Questions.
i. Would you add or subtract the latent time (or lag of the color at
the beginning), if it were known and you wished the true time of lag?
Why? 2. What is the unit of measurement in this exercise?
```

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