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 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.