In any case, something must have gone wrong, his calculations, his cartography, for it was not until after sunrise on the thirtieth, more than two full days behind schedule and only a few hours before the predicted eclipse, that Leibniz reached the observatory, and almost without realizing it, for he was at that point, as he recounts, suffering from hunger, dehydration, and hypothermia (“I have nearly completed a treatise on some remarkable properties of mountains,” he reports to the Philosophical Transactions), and had hallucinated, over the course of the night prior, while staggering from one peak to the next under the lash of a freezing rain, a vast number of astronomical observatories “of every conceivable and inconceivable geometry,” not only triangular observatories and pentagonal observatories but observatories of hundreds of sides and in fact even thousands of sides, “in short, a night of pure polygonal unease.” “Observatories were constantly popping out of the mist, and only after I ran up to them in relief did I realize they were in my head.” So, when the Sun rose and he saw right in front of him a small, perfectly circular tower of red brick, crumbling in parts, perched on the edge of a high cliff well above the clouds, with an immense telescope, even longer than rumored, protruding from it at an angle over the void, he assumed at first (“not unreasonably!”) that this circular observatory was just another illusion produced by his bedeviled mind, the final illusion, the observatory toward which last night’s distressing sequence was inexorably tending, and which, knowing his mind, his mind would not rest until it had produced. (“In the astronomer’s mind I would soon encounter a similar, but of course not identical, since no two things in nature are identical, sort of mind.”) It was only when he heard the telescope creak (“animalistically”) in the wind that Leibniz realized he had actually found it, “it was not inside my head, it was actually out there in the snow on the edge of the cliff.”

On the next page of his account he depicts with a draftsman’s hand the intricate system of pulleys and poles that held the heavy iron instrument aloft, marking lengths and angles with terrific precision. It is an impressive diagram, a marvel of its kind, far more detailed than the diagrams most instrument-makers of the time drew or commissioned for their own contraptions. The caption reads: “It was not inside my head.”