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Published online by Cambridge University Press: 04 November 2009
THE SIGHTING TUBE
The dioptra evolved from the humblest of beginnings to become the surveying instrument par excellence of the Greeks. The word means simply ‘something to look through’, and as such it bears a number of connotations which are quite irrelevant to our purposes – a spy, a translucent mineral such as talc or mica for use in windows, a gynaecologist's vaginal speculum or dilator for internal examination. In a catapult, the dioptra was the window in the main frame through which the arrow was discharged and through which, using it as a foresight, the operator took aim. Dioptra could even be used figuratively, meaning ‘perspicacity’.
It was long appreciated that looking at a distant object through a tube clarifies the vision by cutting out extraneous light and unwanted parts of the field of view. Aristotle remarked that
the man who shades his eyes with his hand or looks through a tube will not distinguish any more or any less the differences of colours, but he will see further … In theory, distant objects would be seen best if a sort of continuous tube extended from the eye to what is observed. The further the tube extends, the greater is bound to be the accuracy with which distant objects can be seen.
He does not call the tube a dioptra.
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