[...Halos

When the eye with normal sight looks at the large letters on the
Snellen test card, at any distance, from twenty feet to six inches or
less, it sees, at the inner and outer edges and in the openings of the
round letters, a white more intense than the margin of the card.
Similarly, when such an eye reads fine print, the spaces between the
lines and the letters and the openings of the letters appear whiter
than the margin of the page, while streaks of an even more intense
white may be seen along the edges of the lines of letters. These
"halos" are sometimes seen so vividly that in order to convince people
that they are illusions it is often necessary to cover the letters,
when they at once disappear. Patients with imperfect sight also see
the halos, though less perfectly, and when they understand that they
are imagined, they often become able to imagine them where they had
not been seen before, or to increase their vividness, in which case
the sight always improves. This can be done by imagining the
appearances first with the eyes closed; and then looking at the card,
or at fine print, and imagining them there. By alternating these two
acts of imagination the sight is often improved rapidly. It is best to
begin the practice at the point at which the halos are seen, or can be
imagined best. Nearsighted patients are usually able to see them at
the near-point, sometimes very vividly. Farsighted people may also see
them best at this point, although their sight for form may be best at
the distance...]

- Dr. W.H. Bates, February 1920