The Mercury Computer (MC) is a magical device (or rather artefact) helping Sailor Mercury in assessing and managing information in real time.
Among its many functions are a decent scanning suite, both optical as well as other venues, but its core functions are in data processing and storage, data manipulation, analysis and more.
The main scanning equipment seems to be located in the visor, though. It is activated by pressing the right earring and slides in front of the eyes. The inherent magic allows it to adapt to various anatomical features of bodies Mercury may inhibit at the time. For instance, when she was possessing an imp, the visor adapted to a form more resembling a helmet. It nevertheless provides a much expanded virtual interface to the computer.
Both are features of Mercury's senshi transformation, but the MC at least can also be accessed in civilian form.