The Heliometer was a split-image micrometer which fit on the objective end of a telescope. It was used to measure angles. It was pretty much the most advanced such instrument for measuring angles at the time of the 1761 and 1769 transits of Venus. Image courtesy of Specula astronomica minima (©Specula astronomica minima).

ToV Reflector with Heliometer 1760s-1770s