spectacle-glass concave on one side and plane on the other. He then applied his eye to the concave glass, and saw objects “pretty large and pretty near him.” They appeared three times nearer, and nine times larger in surface, than to the naked eye. He soon after made another, which represented objects above sixty times larger; and, sparing neither labour nor expense, he finally constructed an instrument so excellent, as “to show things almost a thousand times larger, and above thirty times nearer to the naked eye.” There is, perhaps, no invention that science has presented to man so extraordinary in its nature, and so boundless in its influence, as that of the telescope. To the uninstructed mind, the power of seeing an object a thousand miles distant, as large and nearly as distinct as if it were brought within a mile of the observer, must seem almost miraculous; and to the philosopher, even, who thoroughly comprehends the principles upon which it acts, it must ever appear one of the most elegant applications of science. To have been the first astronomer in whose hands such a gift was placed was a preference to which Galileo owed much of his future reputation. No sooner had he completed his telescope than he applied it to the heavens, and on the 7th January, 1618, the first day of its use, he saw round Jupiter three bright little stars lying in a line parallel to the ecliptic, two to the east, and one to the west of the planet. Regarding them as ordinary stars, he never thought of estimating their distances. On the following day, when he accidentally directed his telescope to Jupiter, he was surprised to see the three stars to the west of the planet. To produce this effect it was requisite that the motion of Jupiter should be direct, though, according to calculation, it was actually retrograde. In this dilemma he waited with impatience for the evening of the 9th, but unfortunately the sky was covered with clouds. On the 10th he saw only two stars to the east—a circumstance which he was no longer able to explain by the motion of Jupiter. He was therefore compelled to ascribe the change to the stars themselves; and upon repeating his observations on the 11th, he no longer doubted that he had discovered three planets revolving round Jupiter. On the 13th January he for the first time saw the fourth satellite.[36]