The experimental work in the direction of a comparison of the optical
illusions with the tactual illusions, to the time of the present
investigation, has been carried on chiefly with the familiar optical
illusion of the overestimation of filled space. If the distance
between two points be divided into two equal parts by a point midway
between them, and the one of the halves be filled with intermediate
points, the filled half will, to the eye, appear longer than the open
half. James[1] says that one may easily prove that with the skin we
underestimate a filled space, 'by taking a visiting card, and cutting
one edge of it into a saw-toothed pattern, and from the opposite edge
cutting out all but two corners, and then comparing the feelings
aroused by the two edges when held against the skin.' He then remarks,
'the skin seems to obey a different law here from the eye.' This
experiment has often been repeated and verified. The most extensive
work on the problem, however, is that by Parrish.[2] It is doubtless
principally on the results of Parrish's experiments that several
authors of text-books in psychology have based their assertions that a
filled space is underestimated by the skin. The opposite conclusion,
namely, that the illusion is not reversed for the skin, has been
maintained by Thiéry,[3] and Dresslar.[4] Thiéry does not, so far as I
know, state the statistics on which he bases his view. Dresslar's
experiments, as Parrish has correctly observed, do not deal with the
proper analogue of the optical illusion for filled space. The work of
Dresslar will be criticised in detail when we come to the illusions
for active touch.

[1] James, William: 'Principles of Psychology,' New York, 1893,
Vol. II., p. 141.

[2] Parrish, C.S.: _Amer. Journ. of Psy._, 1895, Vol. VI., p.