to
perceive the books to stand still. After more trials I sometimes
brought myself to believe, that I saw changing spectra of lights and
shades moving in my eyes, after turning round for some time, but did
not imagine either the spectra or the objects to be in a state of
gyration. I speak, however, with diffidence of these facts, as I could
not always make the experiments succeed, when there was not a strong
light in my room, or when my eyes were not in the most proper state for
such observations.

The ingenious and learned M. Sauvage has mentioned other theories to
account for the apparent circumgyration of objects in vertiginous
people. As the retrograde motions of the particles of blood in the
optic arteries, by spasm, or by fear, as is seen in the tails of
tadpoles, and membranes between the fingers of frogs. Another cause he
thinks may be from the librations to one side, and to the other, of the
crystalline lens in the eye, by means of involuntary actions of the
muscles, which constitute the ciliary process. Both these theories lie
under the same objection as that of Dr. Wells before mentioned; namely,
that the apparent motions of objects, after the observer has revolved
for some time, should appear to vibrate this way and that; and not to
circulate uniformly in a direction contrary to that, in which the
observer had revolved.

M. Sauvage has, lastly, mentioned the theory of colours left in the
eye, which he has termed impressions on the retina. He says,
"Experience teaches us, that impressions made on the retina by a
visible object remain some seconds after the object is removed; as
appears from the circle of fire which we see, when a fire-stick is
whirled round in the dark; therefore when we are carried round our own
axis in a circle, we undergo a temporary vertigo, when we stop; because
the impressions of the circumjacent objects remain for a time
afterwards on the retina." Nosolog. Method. Clas. VIII. I. 1. We have
before observed, that the changes of these colours remaining in the
eye, evinces them to be motions of the fine terminations of the retina,
and not impressions on it; as impressions on a passive substance must
either remain, or cease intirely. See an additional note at the end of
the second volume.

Any one, who stands alone on the top of a high tower, if he ha