d waking, that they observe the room, they sit in, or some of
its furniture, to librate like the motion of the vessel. This I have
experienced myself, and have been told, that after long voyages, it is some
time before these ideas entirely vanish. The same is observable in a less
degree after having travelled some days in a stage coach, and particularly
when we lie down in bed, and compose ourselves to sleep; in this case it is
observable, that the rattling noise of the coach, as well as the undulatory
motion, haunts us. The drunken vertigo, and the vulgar custom of rocking
children, will be considered in the next Section.

6. The motions, which are produced by the power of volition, may be
immediately stopped by the exertion of the same power on the antagonist
muscles; otherwise these with all the other classes of motion continue to
go on, some time after they are excited, as the palpitation of the heart
continues after the object of fear, which occasioned it, is removed. But
this circumstance is in no class of motions more remarkable than in those
dependent on irritation; thus if any one looks at the sun, and then covers
his eyes with his hand, he will for many seconds of time, perceive the
image of the sun marked on his retina: a similar image of all other visible
objects would remain some time formed on the retina, but is extinguished by
the perpetual change of the motions of this nerve in our attention to other
objects. To this must be added, that the longer time any movements have
continued to be excited without fatigue to the organ, the longer will they
continue spontaneously, after the excitement is withdrawn: as the taste of
tobacco in the mouth after a person has been smoaking it.

This taste remains so strong, that if a person continues to draw air
through a tobacco pipe in the dark, after having been smoking some time, he
cannot distinguish whether his pipe be lighted or not.

From these two considerations it appears, that the dizziness felt in the
head, after seeing objects in unusual motion, is no other than a
continuation of the motions of the optic nerve excited by those objects and
which engage our attention. Thus on turning round on one foot, the vertigo
continues for some seconds of time after the person is fallen on the
ground; and the longer he has continued to revolve, the longer will
continue these successive motions of the parts of the optic nerve.

_Additional Observations on _VERTIGO.