the colour-impression.  Thus, santonin, when taken in any quantity, makes all colourless objects look yellow.  Severe pathological disturbances are known to involve, in addition to hyperaesthesia and anaesthesia, what, has been called paraesthesia, that is to say, that condition in which the quality of sensation is greatly changed.  Thus, for example, to one in this state all food appears to have a metallic taste, and so on.

If we now glance back at the various groups of illusions just illustrated, we find that they all have this feature in common:  they depend on the general mental law that when we have to do with the unfrequent, the unimportant, and therefore unattended to, and the exceptional, we employ the ordinary, the familiar, and the well-known as our standard.  Thus, whether we are dealing with sensations that fall below the ordinary limits of our mental experience, or with those which arise in some exceptional state of the organism, we carry the habits formed in the much wider region of average every-day perception with us.  In a word, illusion in these cases always arises through what may, figuratively at least, be described as the application of a rule, valid for the majority of cases, to an exceptional case.

In the varieties of illusion just considered, the circumstance that gives the peculiarity to the case thus wrongly interpreted has been referred to the organism.  In the illusions to which we now pass, it will be referred to the environment.  At the same time, it is plain that there is no very sharp distinction between the two classes.  Thus, the visual illusion produced by pressing the eyeball might be regarded not only as the result of the organic law of the “specific energy” of the nerves, but, with almost equal appropriateness, as the consequence of an exceptional state of things in the environment, namely, the pressure of a body on the retina.  As I have already observed, the classification here adopted is to be viewed simply as a rough expedient for securing something like a systematic review of the phenomena.

CHAPTER V.

ILLUSIONS OF PERCEPTION—­continued.

A. Passive Illusions (b) as determined by the Environment.

In the following groups of illusion we may look away from nervous processes and organic disturbances, regarding the effect of any external stimulus as characteristic, that is, as clearly marked off from the effects of other stimuli, and as constant for the same stimulus.  The source of the illusion will be looked for in something exceptional in the external circumstances, whereby one object or condition of an object imitates the effect of another object or condition, to which, owing to a large preponderance of experience, we at once refer it.

Exceptional Relation of Stimulus to Organ.

A transition from the preceding to the following class of illusions is to be met with in those errors which arise from a very exceptional relation between the stimulus and the organ of sense.  Such a state of things is naturally interpreted by help of more common and familiar relations, and so error arises.