The present volume takes a wide survey of the field of error, embracing
in its view not only the illusions of sense dealt with in treatises on
physiological optics, etc., but also other errors familiarly known as
illusions, and resembling the former in their structure and mode of
origin. I have throughout endeavoured to keep to a strictly scientific
treatment, that is to say, the description and classification of
acknowledged errors, and the explanation of these by a reference to
their psychical and physical conditions. At the same time, I was not
able, at the close of my exposition, to avoid pointing out how the
psychology leads on to the philosophy of the subject. Some of the
chapters were first roughly sketched out in articles published in
magazines and reviews; but these have been not only greatly enlarged,
but, to a considerable extent, rewritten.
J. S.

_Hampstead, April, 1881._




CONTENTS.


CHAPTER I.

THE STUDY OF ILLUSION.

Vulgar idea of Illusion, 1, 2; Psychological treatment of subject,