In the year 1665 the famous astronomer Hooke wrote concerning the small star Gamma Arietis: 'I took notice that it consisted of two small stars very near together; a like instance of which I have not else met with in all the heavens.' This is the first English record of the observation of a double star, though Riccioli detected the duplicity of Zeta Ursæ Majoris (Mizar), in 1650, and Huygens saw three stars in Theta Orionis in 1656. These were the earliest beginnings of double-star observation, which has since grown to such proportions that double stars are now numbered in the heavens by thousands. Of course, certain stars appear to be double even when viewed with the unaided eye. Thus Mizar, a bright star in the handle of the Plough, referred to above, has not far from it a fainter companion known as Alcor, which the Arabs used to consider a test of vision. Either it has brightened in modern times, or else the Arabs have received too much credit for keenness of sight, for Mizar and Alcor now make a pair that is quite easy to very ordinary sight even in our turbid atmosphere. Alpha Capricorni, and Zeta Ceti, with Iota Orionis are also instances of naked-eye doubles, while exceptionally keen sight will detect that the star Epsilon Lyræ, which forms a little triangle with the brilliant Vega and Zeta Lyræ, is double, or at least that it is not single, but slightly elongated in form. Astronomers, however, would not call such objects as these 'double stars' at all; they reserve that title for stars which are very much closer together than the components of a naked-eye double can ever be. The last-mentioned star, Epsilon Lyræ, affords a very good example of the distinction. To the naked eye it is, generally speaking, not to be distinguished from a single star. Keen sight elongates it; exceptionally keen sight divides it into two stars extremely close to one another. But on using even a very moderate telescope, say a 2-1/2-inch with a power of 100 or upwards, the two stars which the keenest sight could barely separate are seen widely apart in the field, while each of them has in its turn split up into two little dots of light. Thus, to the telescope, Epsilon Lyræ is really a quadruple star, while in addition there is a faint star forming a triangle with the two pairs, and a large instrument will reveal two very faint stars, the 'debilissima,' one on either side of the line joining the larger stars. These I have seen with 3-7/8-inch.