This mezzotint shows two servants of George Parker, 2nd Earl of Macclesfield, in the observatory at Shirburn Castle. They are Thomas Phelps, then aged 82, and John Bartlett, then aged 54. The Observatory was erected in about 1739, about 100 yards south of the Castle.

The text below the image describes the two individauals. Phelps, "who from being a stable-boy in the year 1718, to the then Lord Chief Justice Parker, afterwards Earl of Macclesfield, rose by his merit to the upper employments in that family, and at last, for his uncommon genius, was promoted to be observer, in their Observatory, at Shirburn Castle". John Bartlett was "originally a shepherd, in which station he by books and observation acquired such a knowledge in computation, and of the heavenly bodies, as induced the late George, Earl of Macclesfield, to appoint him assistant observer in his Observatory".

Weld's History of the Royal Society states that there was a copy of this engraving at the Royal Astronomical Society, on the back of which is hand-written, "Thomas Phelps was the person, who, on the 23rd December, 1743, discovered the great comet, and made the first observation of it, an account of which is entered in the Philosophical Transactions, but not the name of the observer".

Phelps and Bartlett are shown in the Shirburn observatory's transit room, with Phelps observing through the 5-foot transit telescope, by Jonathan Sisson, fixed to supporting pillars. He is steadying himself by leaning on the back of an observing chair. Behind Bartlett is an astronomical regulator, an accurate observatory clock, by George Graham. To the left is an equatorially-mounted, unfixed telescope, probably by Dollond, and in the right foreground is a celestial globe. Although not shown here, the observatory also had a large mural quadrant, with a scale, like that at Greenwich, divided first by George Graham and then by John Bird, and another clock by Thomas Tompion. It was one of the best and most-expensively furnished observatories in Britain at the time. Regular observations were recorded from the 1740s to 1790s.