This sketch accompanies the first known description of a telescope. It is found in a letter from Giovanni Battista della Porta (c.1535-1615) dated 28th August, 1609 to Federico Cesi (1585-1630), in which he describes a telescope he examined in Naples in the summer of 1609.
The telescope was therefore invented by someone else, before 1609.
Here is della Porta’s description: ‘It is a tube made of silver-plated tin, having a length of a one palm ad, three fingers in diameter, with a convex eyepiece at the end a: there is another tube [c] in the same tube, 4 fingers long, which enters into the first, and has a concave [lens] at the top’.
“È un cannello di stagno di argento, lungo un palmo ad, grosso di tre diti di diametro, che ha nel capo a un occhiale convesso: vi è un altro canal [c] del medesimo, di 4 diti lungo, che entra nel primo, et ha un concavo nella cima”.
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