By MARCIA BARTUSIAK
By Robert Scott Root-Bernstein.
Illustrated. 501 pp. Cambridge, Mass.:
Harvard University Press. $35.
As the legend goes, a stray penicillium mold lands on a bacteria-filled petri dish in Fleming's laboratory. About to discard the dish, Fleming suddenly notices that the mold has dissolved the bacterial colonies. Voila! Antibiotics. But Imp deduces from circumstantial evidence that Fleming merely noticed at first the errant mold's mild antiseptic properties; only after deliberately culturing the mold did he clinch that the bacteria were being wiped out. ''Fleming clearly experimented with anything he could lay his hands on, wherever he found it,'' Imp points out. ''That was part of his research style. Playing.'' Good scientists seem to design experiments that will yield surprises; they foster the conditions. Examinations of a host of notable achievers in science show them to be broadly educated, with more than a passing interest in art, music, poetry and literature. Often making their mark in previously unfashionable or neglected areas of research, they retain a childlike curiosity about the world.