first (the green rays).[34]

These momentary modifications of sensibility are of no practical
significance, being almost instantly corrected. Other modifications are
more permanent. It was found by Himly that when the retina is
overexcitable every stimulus is raised in the spectrum scale of colours.
Thus, violet becomes red. An exactly opposite effect is observed when
the retina is torpid.[35] Certain poisons are known to affect the
quality of 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 hyperæsthesia and
anæsthesia, what, has been called paræsthesia, 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