The most skilful astronomers in their observation of Mercury have been frequently deceived; firstly, those, who in the time of Charlemagne, on the 16th of April in the year 807, believed that the transit of Mercury over the Sun continued eight days: secondly, Averrhoes, who says in the Ptolemaic Paraphrase, that he recollected to have seen something of a darkish appearance, and subsequently found by the numbers that the conjunction of Mercury and the Sun had been predicted; he flourished about the year 1160 of the christian era: thirdly, Kepler himself, the most learned astronomer that ever lived, was greatly deceived on the 18th of May 1607. All these having seen spots on the Sun's disc, an appearance not understood in those days, rashly concluded them to be the planet Mercury; but they were evidently misled, as circumstances afterwards proved.

Are we then similarly deceived, and do we mistake an ordinary spot for Venus?

Verily since this may be doubted, as well by some who are unacquainted with the heavens except from books, as by others who are learned and practical astronomers; and lest our labor should be in vain, it may be worth while, before further prosecuting the enquiry, to prove in a satisfactory manner that the planet Venus was the actual cause of this appearance.