rongly, and to be easily moved both by the imagination and by the
light as often as bright objects are looked upon."--Letter from Sir I.
Newton to Locke, Lord Kinq's Life of Locke, vol. i. pp. 405-408.

Dr. Roget (Animal and Vegetable Physiology considered with reference to
Natural Theology, "Bridgewater Treatise," pp. 524, 525) thus refers to
this phenomenon, which he states "all of us may experience ":--

"When the impressions are very vivid" (Dr. Roget is speaking of visual
impressions), "another phenomenon often takes place,--namely, their
_subsequent recurrence after a certain interval, during which they are
not felt, and quite independently of any renewed application of the
cause which had originally excited them."_ (I mark by italics the words
which more precisely coincide with Julius Faber's explanations.) "If,
for example, we look steadfastly at the sun for a second or two, and
then immediately close our eyes, the image, or spectrum, of the sun
remains for a long time present to the mind, as if the light were still
acting on the retina. It then gradually fades and disappears; but if
we continue to keep the eyes shut, the same impression will, after
a certain time, recur, and again vanish: and this phenomenon will be
repeated at intervals, the sensation becoming fainter at each renewal.
It is probable that these reappearances of the image, after the
light which produced the original impression has been withdrawn, are
occasioned by spontaneous affections of the retina itself which are
conveyed to the sensorium. In other cases, where the impressions are
less strong, the physical changes producing these changes are perhaps
confined to the sensorium."

It may be said that there is this difference between the spectrum of the
sun and such a phantom as that which perplexed Allen Fenwick,--namely,
that the sun has been actually beheld before its visionary appearance
can be reproduced, and that Allen Fenwick only imagines he has seen the
apparition which repeats itself to his fancy. "But there are grounds for
the suspicion" (says Dr. Hibbert, "Philosophy of Apparitions," p. 250),
"that when ideas of vision are vivified to the height of sensation, a
corresponding affection of the optic nerve accompanies the illusion."
Muller ("Physiology of the Senses," p. 1392, Baley's translation) states
the same opinion still more strongly; and Sir David Brewster, quoted
by Dr. Hibbert (p. 251) says: "In examining these mental impress