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impression has been withdrawn, are occasioned by spontaneous affections of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