ity of that part of
the retina which has been exposed to the black spot, than of the other
parts which had received the white field of paper, which is put beyond a
doubt by the next experiment.

4. On closing the eyes after viewing the black spot on the white paper, as
in the foregoing experiment, a red spot is seen of the form of the black
spot: for that part of the retina, on which the black spot was delineated,
being now more sensible to light than the other parts of it, which were
exposed to the white paper, is capable of perceiving the red rays which
penetrate the eyelids. If this experiment be made by the light of a tallow
candle, the spot will be yellow instead of red; for tallow candles abound
much with yellow light, which passes in greater quantity and force through
the eyelids than blue tight; hence the difficulty of distinguishing blue
and green by this kind of candle light. The colour of the spectrum may
possibly vary in the daylight, according to the different colour of the
meridian or the morning or evening light.

M. Beguelin, in the Berlin Memoires, V. II. 1771, observes, that, when he
held a book so that the sun shone upon his half-closed eyelids, the black
letters, which he had long inspected, became red, which must have been thus
occasioned. Those parts of the retina which had received for some time the
black letters, were so much more sensible than those parts which had been
opposed to the white paper, that to the former the red light, which passed
through the eyelids, was perceptible. There is a similar story told, I
think, in de Voltaire's Historical Works, of a Duke of Tuscany, who was
playing at dice with the general of a foreign army, and, believing he saw
bloody spots upon the dice, portended dreadful events, and retired in
confusion. The observer, after looking for a minute on the black spots of a
die, and carelessly closing his eyes, on a bright day; would see the image
of a die with red spots upon it, as above explained.

5. On emerging from a dark cavern, where we have long continued, the light
of a bright day becomes intolerable to the eye for a considerable time,
owing to the excess of sensibility existing in the eye, after having been
long exposed to little or no stimulus. This occasions us immediately to
contract the iris to its smallest aperture, which becomes again gradually
dilated, as the retina becomes accustomed to the greater stimulus of the
daylight.

The twinkling of a brigh