Chapter I: 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. J. S.

_Hampstead, April, 1881._

CONTENTS.

CHAPTER I.

THE STUDY OF ILLUSION.

Vulgar idea of Illusion, 1, 2; Psychological treatment of subject,
3, 4; definition of Illusion, 4-7; Philosophic extension of idea,
7, 8.

CHAPTER II.

THE CLASSIFICATION OF ILLUSIONS.

Popular and Scientific conceptions of Mind, 9, 10; Illusion and
Hallucination, 11-13; varieties of Immediate Knowledge, 13-16; four-fold
division of Illusions, 16-18.

CHAPTER III.

ILLUSIONS OF PERCEPTION: GENERAL.

_Psychology of Perception_:--The Psychological analysis of Perception,
19, 20; Sensation and its discrimination, etc., 20, 21; interpretation
of Sensation, 22, 23; construction of material object, 23, 24;
recognition of object, specific and individual, 24-27; Preperception
and Perception, 27-31; Physiological conditions of Perception, 31-33;
Visual and other Sense-perception, 33, 34.

_Illusions of Perception_:--Illusion of Perception defined, 35-38;
sources of Sense-illusion, 38-40: (a) confusion of
Sense-impression, 40-44; (b) misinterpretation of Sense-impression, 44;
Passive and Active misinterpretation, 44-46; Passive Illusions as
organically and extra-organically conditioned, 46-49.

CHAPTER IV.

ILLUSIONS OF PERCEPTION--_continued_.

A. _Passive Illusions (a) as determined by the Organism._
_Results of Limits of Sensibility_:--Relation of quantity of Sensation to
that of Stimulus, 50-52; coalescence of simultaneous Sensations,
52-55; after-effect of Stimulation, 55, 56; effects of prolonged
Stimulation, 56-58; Specific Energy of Nerves, 58, 59; localization
of Sensation, 59-62; Subjective Sensations, 62-64.

_Results of Variation of Sensibility_:--Rise and fall of Sensibility,
64-67; Paræsesthesia, 67, 68; _rationale_ of organically conditioned
Illusions, 68, 69.

CHAPTER V.

ILLUSIONS OF PERCEPTION--_continued_.

A. _Passive Illusions (b) as determined by the Environment._
_Exceptional Relation of Stimulus to Organ_:--Displacement of organ,
etc., 70-72.

_Exceptional Arrangement of Circumstances in the Environment_:--
Misinterpretation of the direction and movement of objects, 72-75;
misperception of Distance, 75, 76; Illusions of depth, relief, and
solidity in Art, 77-81; Illusions connected with the perception of
objects through transparent coloured media, 82-84; visual transformation
of concave into convex form, 84-86; false recognition of
objects, 86, 87; inattention to Sense-impression in Recognition,
87-91; suggestion taking the direction of familiar recurring experiences,
91, 92.

CHAPTER VI.

ILLUSIONS OF PERCEPTION--_continued_.

B. _Active Illusions._

Preperception and Illusion, 93-95.

_Voluntary Preperception_:--Choice of interpretation in the case of
visible movement, 95, 96; and in the case of flat projections of form,
96-98; capricious interpretation of obscure impressions, 99, 100.

_Involuntary Preperception_:--Effects of permanent Predisposition,
101, 102; effects of partial temporary Preadjustment, 102-105;
complete Pro-adjustment or Expectation, 106-109; subordination of
Sense-impression to Preperception, 109-111; transition from Illusion
to Hallucination, 111, 112; rudimentary Hallucinations, 112-114;
developed Hallucinations, 114-116; Hallucination in normal life,
116, 117; Hallucinations of insanity, 118-120; gradual development
of Sense-illusions, and continuity of normal and abnormal life; 120-123;
Sanity and Insanity distinguished, 123-126.