Page:Galileo (1918).djvu/23

Rh Galileo first saw three bright objects close to the planet. Repeated observation proved that these, with a fourth, which must have been on the first night hidden by the planet, were moons revolving round Jupiter. This conclusion was reached in a very few weeks, and Galileo gave the newly discovered bodies the name of "Medicean stars" in compliment to the Grand Duke and his three brothers.

Meanwhile Galileo had fitted up a workshop for the manufacture of telescopes, finding, however, that only about one in ten of his object glasses was good enough to show the "Medicean stars". The demand for the new instrument was great, as Galileo's telescopes were much more efficient than any to be procured elsewhere, so that his workmen, who made also hundreds of his other instruments, geometrical compasses, hydrostatic balances, air thermometers, magnets, and so on, were kept busy, though Galileo ground all his lenses himself until his sight failed. It was some time before the superiority of other forms of telescope for very high magnifying power was realised. Galileo's principle still survives in field glasses, where the power is not high and when the inverting effect of the astronomical telescope is inconvenient, especially as the erecting eyepiece involves loss of light. The Grand Duke Cosmo II. asked for the telescope with which these discoveries were made, but Galileo, though he at once consented, nevertheless kept it for his own use, ostensibly only on loan. Its focal length was 5$1⁄2$ feet and aperture 2$1⁄4$ inches.

During the earlier part of 1610 more than a hundred telescopes, accompanied by copies of the "Sidereus Nuncius," were sent to princes and learned men of Italy, France, Flanders, and Germany, and the French Court in particular, where Marie dei Medici was queen, desired to bespeak for the King, Henry of Bourbon, the privilege of