During a transit, Venus appears as a small dark circle on the face of the Sun. Although it can be seen with the (suitably protected) naked eye, a telescope is essential to make useful observations of its motion and position.
The favoured instruments of the major British expeditions of the 18th century were reflecting telescopes, which use a mirror rather than a lens as the primary optical component. The preferred supplier was James Short, who specialised exclusively in the manufacture of reflectors.
The telescopes were used together with an expeditions clocks to see and time the key moments of a transit. By adding a micrometer, a telescope could also be converted into a measuring instrument, so that small angles such as the diameter of Venus could be accurately determined.