It is worth adding, perhaps, that these variations in sensibility give
rise not only to sensory but also to motor illusions. To take a homely
instance, the last miles of a long walk seem much longer than the first,
not only because the sense of fatigue leading us to dwell on the
transition of time tends to magnify the apparent duration, but because
the fatigued muscles and connected nerves yield a new set of sensations
which constitute an exaggerated standard of measurement. A number of
optical illusions illustrate the same thing. Our visual sense of
direction is determined in part by the feelings accompanying the action
of the ocular muscles, and so is closely connected with the perception
of movement, which has already been touched on. If an ocular muscle is
partially paralyzed it takes a much greater "effort" to effect a given
extent of movement than when the muscle is sound. Hence any movement
performed by the eye seems exaggerated. Hence, too, in this condition
objects are seen in a wrong direction; for the patient reasons that they
are where they would seem to be if he had executed a wider movement than
he really has. This may easily be proved by asking him to try to seize
the object with, his hand. The effect is exaggerated when complete
paralysis sets in, and no actual movement occurs in obedience to the
impulse from within.[33]

Variations in the condition of the nerve affect not only the degree, but
also the quality of the sensation, and this fact gives rise to a new
kind of illusion. The curious phenomena of colour-contrast illustrate
momentary alterations of sensibility. When, after looking at a green
colour for a time, I turn my eye to a grey surface and see this of the
complementary rose-red hue, the effect is supposed to be due to a
temporary fatigue of the retina in relation to those ingredients of the
total light in the second case which answer to the partial light in the