which he constructed in 1602. No doubt this apparatus in its
primitive form differed in some respects from the contrivance we call
by the same name. Galileo at first employed water as the agent, by
the expansion of which the temperature was to be measured. He
afterwards saw the advantage of using spirits for the same purpose.
It was not until about half a century later that mercury came to be
recognised as the liquid most generally suitable for the thermometer.

The time was now approaching when Galileo was to make that mighty
step in the advancement of human knowledge which followed on the
application of the telescope to astronomy. As to how his idea of
such an instrument originated, we had best let him tell us in his own
words. The passage is given in a letter which he writes to his
brother-in-law, Landucci.

"I write now because I have a piece of news for you, though whether
you will be glad or sorry to hear it I cannot say; for I have now no
hope of returning to my own country, though the occurrence which has
destroyed that hope has had results both useful and honourable. You
must know, then, that two months ago there was a report spread here
that in Flanders some one had presented to Count Maurice of Nassau a
glass manufactured in such a way as to make distant objects appear
very near, so that a man at the distance of two miles could be
clearly seen. This seemed to me so marvellous that I began to think
about it. As it appeared to me to have a foundation in the Theory of
Perspective, I set about contriving how to make it, and at length I
found out, and have succeeded so well that the one I have made is far
superior to the Dutch telescope. It was reported in Venice that I
had made one, and a week since I was commanded to show it to his
Serenity and to all the members of the senate, to their infinite