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Picard's observations at Paris from 1667 onwards seem to embody the first use of the telescope as a pointer. He was also the first to introduce the use of Huyghens's clocks for observing the right ascension of stars. Olaus Romer was born at Copenhagen in 1644. In 1675, by careful study of the times of eclipses of Jupiter's satellites, he discovered that light took time to traverse space.

Thus the startling question is suggested, Why may not the ultimate atoms of matter be vortex-rings forever existing in such a frictionless fluid filling the whole of space? Such a hypothesis is not less brilliant than Huyghens's conjectural identification of light with undulatory motion; and it is moreover a legitimate hypothesis, since it can be brought to the test of verification.

This theory explained all the phases of the ring which had puzzled others. This ring was then, and has remained ever since, a unique structure. We in this age have got accustomed to it. But Huyghens's discovery was received with amazement.

One of these last is nearly identical with the armillae of Tycho; and the other with his "armillae aequatoriae maximae," with which he observed the comet of 1585, besides fixed stars and planets. The discovery by Galileo of the isochronism of the pendulum, followed by Huyghens's adaptation of that principle to clocks, has been one of the greatest aids to accurate observation.

It was thus in sundry of the cases we have quoted as at variance with M. Comte's doctrine. It was thus with the application of Huyghens's optical discovery to astronomical observation by Galileo. It was thus with the application of the isochronism of the pendulum to the making of instruments for measuring intervals, astronomical and other.