at any stage a dark interval between the corona and the disk ; more probably the inner edge of the corona was slightly within the apparent permanent boundary of the disk. The red corona was very narrow; its greatest width appeared to be rather less than 1 mm., or about one-fifth of a degree. The effect was best seen when the attention was directed upon the tinfoil strip, which for a moment, after the exposure, became bright red, the coronae, or red borders, of the adjoining semi-disks meeting or perhaps overlapping one another. The apparent temporary excess of the area of the disk above its final area, as mentioned in (1), was probably due to the evanescent red border.[1]
It is remarkable that repeated experiments had been made with this and similar apparatus for several weeks before the existence of the red border was detected, even though something of the kind was looked for. The difficulty is, not to see it, but to know that one sees i t ; when once it has been perceived it becomes very conspicuous. The phenomenon is beyond doubt constantly met with, and habitually ignored, in daily life. Since my first observation of it I have many times noticed flashes of red upon the black letters of a book or upon the edges of the page; bright metallic or polished objects often show it when they pass across the field of vision in consequence of a movement of the eyes, and it was an accidental observation of this kind that suggested the following experiment.
Experiment II.
(1) The zinc plate of the last experiment was taken from the box, and the aperture in the plate was covered with thin paper. A ground glass lamp of 8-candle power, attached to a flexible cord, was put behind it, and the whole was moved rather quickly either backwards and forwards or round and round in a small circle at a distance of a foot or so from the eyes. The edges of the straight or circular streak of light thus formed were bordered with red.
(2) A 16-candle power lamp was substituted for the other. The red border then appeared to have a greenish-blue band inside it, slightly encroaching upon the streak of light; probably, however, it was only the apparent or irradiation boundary that was thus affected, not the true geometrical boundary.
(3) The paper was removed, and the 8-candle power ground-glass lamp was again placed behind the aperture. The red could now no longer be seen, but the greenish-bine border remained.
(4) When the 16-candle power lamp was used in the same way
- ↑ The effect may be seen without the use of the spring shutter, if a black screen be held before the eyes and suddenly removed, but it is more difficult to hit upon the exact position of the disk.
