lasses
being distant from each other by the difference of their focal lengths.
An exactly similar principle is adopted in the construction of an
opera-glass, which can be accurately described as a double Galilean
telescope. Galileo must be regarded as the inventor of this kind of
telescope, which in one respect differed very materially from the one
constructed by the Dutch optician. If what has been said with regard to
the _inverted_ weather-cock be true, then Lippershey's telescope was
made with two convex lenses, distant from each other by the sum of their
focal lengths, and all objects observed with it were seen inverted.
Refracting astronomical telescopes are now constructed on this
principle, it having been discovered that for observational purposes
they possess several advantages over the Galilean instrument. When
Galileo had completed his first telescope he returned with it to
Venice, where he exhibited it to his friends. The sensation created by
this small instrument, which magnified only three times, was most
extraordinary, and almost amounted to a frenzy. Crowds of the principal
citizens of Venice flocked to Galileo's house in order that they might
see the magical tube about which such wonderful reports were circulated;
and for upwards of a month he was daily occupied in describing his
invention to attentive audiences. At the expiration of this time the
Doge of Venice, Leonardo Deodati, hinted that the Senate would not be
averse to receive the telescope as a gift. Galileo readily acquiesced
with this desire, and, as an acknowledgment of his merits, a decree was
issued confirming his appointment as professor at Padua for life, and
increasing his salary from 500 to 1,000 florins. The public excitement
created by the telescope showed no signs of abatement. Sirturi mentions
that, having succeeded in constructing an instrument, he ascended the
tower of St. Mark's at Venice, hoping to be able to use it there without
interruption. He was, however, detected by a few individuals, and soon
surrounded by a crowd, which took possession of his telescope, and
detained him for several hours until their curiosity was satisfied.
Eager inquiries having been made as to where he lodged, Sirturi, fearing
a repetition of his experience in the church tower, decided to quit
Venice early next morning, and betake himself to a quieter and less
frequented neighbourhood.

The instrument was at first called Galileo's tube; the double eye-g