Besides these contrivances for measuring time during the day, it is almost certain that they must have possessed means of measuring time during the night.

The clepsydra, or water-clock, which was in common use among the Greeks as early as the fifth century before our era, was probably introduced into Greece from the East, and is likely to have been a Babylonian invention.

The astrolabe, an instrument for measuring the altitude of stars above the horizon, which was known to Ptolemy, may also reasonably be assigned to them.

It has generally been assumed that they were wholly ignorant of the telescope.
But if the satellites of Saturn are really mentioned, as it is thought that they are, upon some of the tablets, it will follow--strange as it may seem to us--that the Babylonians possessed optical instruments of the nature of telescopes, since it is impossible, even in the clear and vapor-loss sky of Chaldaea, to discern the faint moons of that distant planet without lenses.

A lens, it must be remembered, with a fair magnifying power, has been discovered among the Mesopotamian ruins.
A people ingenious enough to discover the magnifying-glass would be naturally led on to the invention of its opposite.