"visual," "auditory," etc., already mentioned. The early examination
of children by this method would probably be of great service in
determining proper courses of treatment, subjects of study, modes of
discipline, tendencies to fatigue and embarrassment, and the direction
of best progress in education.

This research may be taken to illustrate the use of the reaction-time
method in investigating such complex processes as attention,
temperament, etc. The department which includes the various time
measurements in psychology is now called Mental Chronometry, the older
term, Psychometry, being less used on account of its ambiguity.

III. _An Optical Illusion._--In the sphere of vision many very
interesting facts are constantly coming to light. Sight is the most
complex of the senses, the most easily deranged, and, withal, the most
necessary to our normal existence. The report of the following
experimental study will have the greater utility, since, apart from
any intrinsic novelty or importance the results may prove to have, it
shows some of the general bearings of the facts of vision in relation
to AEsthetics, to the theory of Illusions, and to the function of
Judgment.

Illusion of the senses is due either to purely physiological causes or
to the operation of the principle of Assimilation, which has already
been remarked upon. In the latter case it illustrates the fact that at
any time there is a general disposition of the mind to look upon a
thing under certain forms, patterns, etc., to which it has grown
accustomed; and to do this it is led sometimes to distort what it sees
or hears unconsciously to itself. So it falls into errors of judgment
through the trap which is set by its own manner of working. Nowhere is
the matter better illustrated than in the sphere of vision. The number
of illusions of vision is remarkable. We are constantly taking shapes
and forms for something slightly different from what, by measurement,
we actually find them to be. And psychologists are attempting--with
rather poor success so far--to find some general principles of the
mechanism of vision which will account for the great variety of its
illusions.

Among these principles one is known as Contrast. It is hardly a
principle as yet. It is rather a word used to cover all illusions
which spring up when surfaces of different sizes and shapes, looked at
together or successively, are misjudged with reference to one another.
Wishing to inve