When the finger-tip was drawn over two adjacent open spaces, and during the first a bell was tapped continuously, this kind of filled space was underestimated if the distance was long and overestimated if the distance was short.  So, too, if a disagreeable odor was held to the nostrils while the finger-tip was being drawn over one of the two adjacent open spaces, the space thus filled by the sensations of smell followed the law already stated.  But if an agreeable perfume was used, the distance always seemed shorter than when an unpleasant odor was given.

In all of these experiments with spaces filled by means of other than tactual sensations, I always compared the judgment on the filled and open spaces with judgments on two open spaces, in order to guard against any error due to unsymmetrical, subjective conditions for the two spaces.  It is difficult to have the subject so seat himself before the apparatus as to avoid the errors arising from tension and flexion.  In one experiment, a piece of plush was used for the filled space and the finger drawn over it against the nap.  This filled space was judged longer than a piece of silk of equal length.  The sensations from the plush were very unpleasant.  One subject said, even, that they made him shudder.  This was of course precisely what was wanted for the experiment.  It showed that the affective tone of the sensation within the filled space was a most important factor in producing an illusory judgment of distance.

The overestimation of these filled spaces is evidently due in a large measure to aesthetic motives.  The space that is filled with agreeable sensations is judged shorter than one which is filled with disagreeable sensations.  In other words, the illusions in judgments on cutaneous space are not so much dependent on the quality of sensations that we get from the outer world through these channels, as from the amount of inner activity that we set over against these bare sense-perceptions.

I have already spoken of the defects of this method of measuring off equivalent distances as a means of getting at the quantitative amount of the illusion.  The results that have come to light thus far have, however, amply justified the method.  I had no difficulty, however, in adapting my apparatus to the other way of getting the judgments.  I had a short curved piece of wire inserted in the handle, which could be held across the line traversed, and thus the end of the open space could be marked out.  Different lengths were presented to the subject as before, but now the subject passed his finger in a uniform motion over the spaces, after which he pronounced the judgment ‘greater,’ ‘equal,’ or ‘less.’  The general result of these experiments was not different from those already given.  The short, filled spaces were overestimated, while the longer ones were underestimated.  The only difference was found to be that now the transition from one direction to the other was at a more distant point.  It was, of course, more difficult to convert these qualitative results into a quantitative determination of the illusion.