by the
battement, or undulations of sound in the ears, which many vertiginous
people experience; and though there is also a tangible vertigo, as when
a blind person turns round, as mentioned above; yet as this
circumgyration of objects is an hallucination or deception of the sense
of sight, we are to look for the cause of our appearing to move
forward, when we stop with our eyes closed after gyration, to some
affection of this sense. Now, thirdly, if the spectra formed in the eye
during our rotation, continue to change, when we stand still, like the
spectra described in Sect. III. 3. 6. such changes must suggest to us
the idea or sensation of our still continuing to turn round; as is the
case, when we revolve in a light room, and close our eyes before we
stop. And lastly, on opening our eyes in the situation above described,
the objects we chance to view amid these changing spectra in the eye,
must seem to move in a contrary direction; as the moon sometimes
appears to move retrograde, when swift-gliding clouds are passing
forwards so much nearer the eye of the beholder.

To make observations on faint ocular spectra requires some degree of
habit, and composure of mind, and even patience; some of those
described in Sect. XL. were found difficult to see, by many, who tried
them; now it happens, that the mind, during the confusion of vertigo,
when all the other irritative tribes of motion, as well as those of
vision, are in some degree disturbed, together with the fear of
falling, is in a very unfit state for the contemplation of such weak
sensations, as are occasioned by faint ocular spectra. Yet after
frequently revolving, both with my eyes closed, and with them open, and
attending to the spectra remaining in them, by shading the light from
my eyelids more or less with my hand, I at length ceased to have the
idea of going forward, after I stopped with my eyes closed; and saw
changing spectra in my eyes, which seemed to move, as it were, over the
field of vision; till at length, by repeated trials on sunny days, I
persuaded myself, on opening my eyes, after revolving some time, on a
shelf of gilded books in my library, that I could perceive the spectra
in my eyes move forwards over one or two of the books, like the vapours
in the air of a summer's day; and could so far undeceive myself, as