A web article with an illustration. I can't find it. But from "Purkinje's Vision: Dawn of Neuroscience"
"The first phenomenon described by Purkinje was novel: He produced flicker, while
looking at the bright sky, by waving his fingers in front of one eye and reported seeing
checkerboards, zigzags, spirals, and ray patterns (see Fig. 3.2, Figs. 1–4). They were
called shadow figures by Helmholtz (1867, 2000b) and are now referred to as
stroboscopic patterns (Smythies, 1957), although they were described before the
stroboscope was invented."
"The first phenomenon described by Purkinje was novel: He produced flicker, while looking at the bright sky, by waving his fingers in front of one eye and reported seeing checkerboards, zigzags, spirals, and ray patterns (see Fig. 3.2, Figs. 1–4). They were called shadow figures by Helmholtz (1867, 2000b) and are now referred to as stroboscopic patterns (Smythies, 1957), although they were described before the stroboscope was invented."