the longer filled distances (which could not be conveniently
reproduced here) the spaces were at first underestimated, and then
this underestimation slowly decreased.

[Illustration: FIG. 8.]

[Illustration: FIG. 9.]

None of the qualitative studies that have hitherto been made on this
illusion have brought to light this significant wearing away of the
illusion.


VII.


I have already spoken of the defects of the apparatus with which the
experiments of the previous chapter were made. I shall now give an
account of some experiments that were made with an apparatus designed
to overcome these difficulties. This is shown in Fig. 10. The block
_C_ was clamped to a table, while the block _A_ could be moved back
and forth by the lever _B_, in order to bring up different lengths of
filled space for judgment. For each judgment the subject brought his
finger back to the strip _D_, and by moving his finger up along the
edge of this strip he always came into contact with the first point of
the new distance. The lever was not used in the present experiment;
but in later experiments, where the points were moved under the finger
tip, which was held stationary, this lever was very useful in
producing different rates of speed. In one series of experiments with
this apparatus the filled spaces were presented first, and in another