These experiments were carried on in the Harvard Psychological Laboratory during the greater part of the years 1898-1901.  In all, fifteen subjects cooeperated in the work at different times.

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 Thiery,[3] and Dresslar.[4] Thiery 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.
   514.

   [3] Thiery, A.:  Philos.  Studien, 1896, Bd.  XII., S. 121.

   [4] Dresslar, F.B.:  Amer.  Journ. of Psy., 1894, Vol.  VI., p.
   332.

At the beginning of the present investigation, the preponderance of testimony was found to be in favor of the view that filled space is underestimated by the skin; and this view is invariably accompanied by the conclusion, which seems quite properly to follow from it, that the skin and the eye do not function alike in our perception of space.  I began my work, however, in the belief that there was lurking somewhere in the earlier experiments a radical error or oversight.  I may say here, parenthetically, that I see no reason why experimental psychologists should so often be reluctant to admit that they begin certain investigations with preconceptions in favor of the theory which they ultimately defend by the results of their experiments.  The conclusions