objects, as trees, etc., and turn our eyes to an apparently stationary
object, as the carpet of the compartment, this seems to move in the
contrary direction to that of the trees. Helmholtz's explanation of this
illusion is that when we suppose that we are fixing our eye on the
carpet we are really continuing to move it over the surface by reason of
the organic tendency, already spoken of, to go on doing anything that
has been done. But since we are unaware of this prolonged series of
ocular movements, the muscular feelings having become faint, we take the
impression produced by the sliding of the picture over the retina to be
the result of a movement of the object.[26]

Another limit to our sensibility, which needs to be just touched on
here, is known by the name of the specific energy of the nerves. One and
the same nerve-fibre always reacts in a precisely similar way, whatever
the nature of the stimulus. Thus, when the optic nerve is stimulated in
any manner, whether by light, mechanical pressure, or an electric
current, the same effect, a sensation of light, follows.[27] In a usual
way, a given class of nerve-fibre is only stimulated by one kind of
stimulus. Thus, the retina, in ordinary circumstances, is stimulated by
light. Owing to this fact, there has arisen a deeply organized habit of
translating the impression in one particular way. Thus, I instinctively
regard a sensation received by means of the optic nerve as one caused by
light.

Accordingly, whenever circumstances arise in which a like sensation is
produced by another kind of stimulus, we fall into illusion. The
phosphenes, or circles of light which are seen when the hinder part of
the eyeball is pressed, may be said to be illusory in so far as we
speak of them as perceptions of light, thus referring them to the
external physical agency which usually causes them. The same remark