The observation you mention . . . I once made upon myself with the hazard of my eyes. The manner was this. I looked a very little while upon the Sun in a looking-glass with my right eye & then turned my eyes into a dark corner of my chamber & winked to observe the impression made & the circles of colours which encompassed it & how they decayed by degrees & at last vanished.
This I repeated a second & a third time. At the third time, when the phantasm of light & colours about it were almost vanished, intending my fancy upon them to see their last appearance I found to my amazement that they began to return & by little & little to become as lively & vivid as when I had newly looked upon the Sun. But when I ceased to intend my fancy upon them they vanished again.
After this I found that as often as I went into the dark & intended my mind upon them as when a man looks earnestly to see anything which is difficult to be seen, I could make the phantasm return without looking any more upon the Sun. And the oftener I made it return, the more easily I could make it return again. And at length by repeating this without looking any more upon the Sun I made such an impression on my eye that if I looked upon the clouds or a book or any bright object I saw upon it a round bright spot of light like the Sun. And, which is still stranger, though I looked upon the Sun with my right eye only & not with my left, that my fancy began to make the impression upon my left eye as well as upon my right. For if I shut my right eye & looked upon a book or the clouds with my left eye I could see the spectrum of the Sun almost as plain as with my right eye, if I did but intend my fancy a little while upon it. For at first if I shut my right eye & looked with my left, the spectrum of the Sun did not appear till I intended my fancy upon it; but by repeating this, appeared every time more easily.
And now in a few hours time I had brought my eyes to such a pass that I could look upon no bright object with either eye but I saw the Sun before me, so that I durst neither write nor read but to recover the use of my eyes shut myself up in my chamber made dark for three days together & used all means to divert my imagination from the Sun.
For if I thought upon him I presently saw his picture though I was in the dark. But by keeping in the dark & employing my mind about other things I began in three or four days to have some use of my eyes again & by forbearing a few days longer to look upon bright objects recovered them pretty well, though not so well but that for some months after the spectrum of the Sun began to return as often as I began to meditate upon the phenomenon, even though I lay in bed at midnight with my curtains drawn. But now I have been very well for many years, though I am apt to think that if I durst venture my eyes I could still make the phantasm return by the power of my fancy.

- Isaac Newton, Letter to John Locke, 1692