Again, by the influx of Mahometans into Europe during five or six centuries, eastern luxuries had been introduced to an extent hitherto unknown, and had in fact become necessities in Christian courts, thus making the India trade the great field of commercial enterprise even by the tedious and uncertain overland routes where middle-men absorbed the profits, and rendering the opening of other and easier routes an object of primary importance. The almost exclusive possession of trade _via_ the old routes by the Italians, furnished an additional motive to other European nations for explorations by sea. The art of printing, recently invented, facilitated the diffusion of learning, so that it was impossible for the world ever again to lapse into the old intellectual darkness. The astrolabe, the foundation of the modern quadrant, had been adapted by a meeting of cosmographers in Portugal to the observation of latitudes by the sun's altitude, and thus the chief obstacle to long sea-voyages was removed. The polarity of the magnet had long been known, but the practical adaptation of the magnetic needle to purposes of navigation occurred about the beginning of the fourteenth century. The mariner's compass, however, only attained its highest purpose toward the close of the fifteenth century, when the Sea of Darkness was traversed. But before this, the greatest impediments to ocean navigation had been overcome by voyages actually made through the aid of the new inventions. Beside the coasts brought to light by these voyages, they had done much to dispel the old superstitions of burning zones, impassable capes, and unnavigable seas. [Sidenote: REAL AND IMAGINARY ISLANDS.]