When Pliny declared in reference to Hipparchus, the ancient astronomer, ‘_Ausus rem Deo improbam annumerare posteris stellas_,’--that ‘he dared to enumerate the stars for posterity, an undertaking forbidden by God,’ what would that natural historian have said, had it been foretold that in less than 1600 years afterwards, a man would arise who should enable posterity to perceive, and to enumerate ten times more new stars than Hipparchus ever beheld--who should point out higher mountains on the moon than on the earth, who should discover dark spots, as large as our globe, in the sun, the fountain of light--who should descry four moons revolving in different periods of time around the planet Jupiter, and could show to surrounding senators the varying phases of Venus? and that another would soon after arise who should point out a double ring of six hundred thousand miles in circumference, revolving around the planet Saturn, and ten hundreds of thousands of stars which neither Hipparchus nor any of the ancient astronomers could ever descry? Yet these are only a small portion of the discoveries made by Galileo and Herschel, by means of the telescope. Had any one prophetically informed Archimedes, the celebrated geometrician of Syracuse, that vision would, in after ages, be thus wonderfully assisted by art--and further, that one manner of improving vision would be to place a dark _opake_ body directly between the object and the eye--and that another method would be, not to look at the object, but to keep the eye quite in a different, and even in an _opposite_ direction, or to stand with the back directly opposed to it, and to behold all the parts of it, invisible to the naked eye, most distinctly in this way--he would, doubtless have considered the prophet as an enthusiastic fool or a raving madman. Yet these things have been realized in modern times in the fullest extent. In the Gregorian reflecting telescope an opake body, namely the small speculum near the end of the tube, interposes _directly_ between the eye and the object. In the Newtonian Reflector, and in the diagonal eye-pieces formerly described, the eye is directed in a line at right angles to the object, or a deviation of 90 degrees from the direct line of vision. In Herschel’s’ large telescopes, and in the _Aerial Reflector_ formerly described (in pp. 311-325) the back is turned to the object, and the eye in an opposite direction.