Mills built his own private observatory on the slopes of Dundee Law, near what is now Adelaide Place. An old print still exists showing the ruins of the building minus its dome. There would appear to have been, in addition to the main telescope, a transit room, and what was probably a study to record and write up his observations. The fact that he had a transit instrument signifies that he must have been doing timings of the passage of stars across the meridian, and was not just a casual observer. One of John Mills' telescope is on display in the Visitor Centre attached to the Royal Observatory, Edinburgh. It is a brass instrument manufactured by George Lowden, a Dundee instrument-maker of that period, who supplied Mills with a number of his instruments.