Persistence of vision: a star drawn into a fiery line
Turning the eye from a star traces a curved fiery streak; a diagram labels a, b, c, d
A note and a labelled diagram treat the lingering of the visual image. If the eye gazing at a star turns quickly the other way, the star seems to draw itself into a curved fiery line. In the diagram, a b c is the lens of the eye and d the star; as the lens sweeps from a to c, the point b arriving at a leaves a continuous line of the star's color, because the eye keeps the image of a shining thing longer than the motion lasts. A drawing at the lower left shows the hatched eye with rays running out to a small star at the upper right.
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A star drawn into a curved fiery line by the turning eye
If the eye that gazes at a star turns quickly to the opposite side, it will seem that the star composes itself into a curved fiery line. The moving eye smears the point of light into a streak.
Diagram a, b, c (the lens) and star d: the lingering image
Let a b c be the lens of the eye that gazes at star d. If the lens sweeps the part a quickly into c, then b, arriving at place a, imprints a continuous line of the star's color, because the eye keeps the image of a shining thing for a while. Such an impression of the star's splendor is more lasting in the pupil than the time of its motion, so it endures across all the positions passing opposite the star.
The star d as the fixed light source
The point of light traced by the eye is a star, marked d in the diagram. It stands opposite the eye's lens as the fixed luminous body whose image is smeared into a line by the eye's quick motion.