sun, in making some of the
preceding experiments, till the disks faded into a pale blue, I frequently
observed a bright blue spectrum of the sun on other objects all the next
and the succeeding day, which constantly occurred when I attended to it,
and frequently when I did not previously attend to it. When I closed and
covered my eyes, this appeared of a dull yellow; and at other times mixed
with the colours of other objects on which it was thrown. It may be
imagined, that this part of the retina was become insensible to white
light, and thence a bluish spectrum became visible on all luminous objects;
but as a yellowish spectrum was also seen in the closed and covered eye,
there can remain no doubt of this being the spectrum of the sun. A similar
appearance was observed by M. AEpinus, which he acknowledges he could give
no account of. (Nov. Com. Petrop. V. 10. p. 2. and 6.)

The locked jaw, and some cataleptic spasms, are resembled by this
phenomenon; and from hence we may learn the danger to the eye by inspecting
very luminous objects too long a time.

IX. _A quantity of stimulus greater than the preceding induces a temporary
paralysis of the organ of vision._

1. Place a circular piece of bright red silk, about half an inch in
diameter, on the middle of a sheet of white paper; lay them on the floor in
a bright sunshine, and fixing your eyes steadily on the center of the red
circle, for three or four minutes, at the distance of four or six feet from
the object, the red silk will gradually become paler, and finally cease to
appear red at all.

2. Similar to these are many other animal facts; as purges, opiates, and
even poisons, and contagious matter, cease to stimulate our system, after
we have been habituated to their use. So some people sleep undisturbed by a
clock, or even by a forge hammer in their neighbourhood: and not only
continued irritations, but violent exertions of any kind, are succeeded by
temporary paralysis. The arm drops down after violent action, and continues
for a time useless; and it is probable, that those who have perished
suddenly in swimming, or in scating on the ice, have owed their deaths to
the paralysis, or extreme fatigue, which succeeds every violent and
continued exertion.

X. MISCELLANEOUS REMARKS.

There were some circumstances occurred in making these experiments, which
were liable to alter the results of them, and which I shall here mention
for the assistance of others, who may