in Optics, an instrument invented by the late learned Dr S. Gravelande, who gave it this name from its fixing, as it were, the rays of the sun in an horizontal direction across the dark chamber all the while it is in use. See Optics Index.

HELIOTROPE (heliotropium), among the ancients; an instrument or machine for showing when the sun arrived at the tropics and the equinoctial line. This name was also used for a sun-dial in general.

HELIOTROPE is also a precious stone, of a green colour, streaked with red veins. Pliny says it is thus called, because, when cast into a vessel of water, the sun's rays falling thereon seem to be of a blood-colour; and that, when out of the water, it gives a faint reflection of the figure of the sun; and is proper to observe eclipses of the sun as a heliometer. The heliotrope is also called oriental jasper, on account of its ruddy spots. It is found in the East Indies, as also in Ethiopia, Germany, Bohemia, &c. Some have ascribed to it the faculty of rendering people invisible, like Gyges's ring.