Source : Webster's Revised Unabridged Dictionary (1913)

Heliometer \He`li*om"e*ter\, n. [Helio- + -meter: cf. F.
   h['e]liom[`e]tre.] (Astron.)
   An instrument devised originally for measuring the diameter
   of the sun; now employed for delicate measurements of the
   distance and relative direction of two stars too far apart to
   be easily measured in the field of view of an ordinary
   telescope.