Mathematics
Astronomers use telescopes and laboratory scientists use microscopes to aid them in seeing more clearly the subjects of their research. For the same purpose mathematicians sometimes use mathescopes. But a mathescope is not a physical instrument. It is an intellectual instrument, with reason for its pedestal and inspiration for its lenses. No one has ever seen a mathescope. But then, neither has anyone ever seen an integral, or a geometric point, or, for that matter, a number. You can see the symbol that someone writes down to stand for a number; but a number itself has no earthly physical being to see, touch, or smell.