Hadley, John H. (1682–1744)
English scientist and mathematician who made improvements in reflecting
telescope design and invented the reflecting quadrant – a forerunner
of the mariner's sextant. Hadley was the
first to devise a laboratory method for testing the parabolic figure of
a mirror. He placed a tiny illuminated pinhole at the mirror's center of
curvature and examined the reflected cone of light in the vicinity of the
image. From the appearance of this cone, he could infer the state of the
mirror's surface and was thus able to pass, by successive polishings, from
a spherical to a paraboloidal figure.
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