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PREFACE.

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.

Hampstead, April, 1881.

J. S.