A double star is one that divides into two with the help of a more or less powerful telescope. The effect is a strange, and might have appeared beforehand a most unlikely one. Yet it is of quite ordinary occurrence. Double stars are no freak of nature, but part of her settled plan; or rather, they enter systematically into the design of the Mind which is in and above nature.

The first recognised specimen of the class was ζ Ursæ Majoris, the middle ‘horse’ of the Plough, called by the Arabs ‘Mizar,’ which Riccioli found at Bologna, in 1650, to consist of a 2½ and a 4 magnitude star within fourteen seconds of arc of each other. Both are radiantly white, and they make a glorious object even in a very small telescope. The accident of a bright comet passing, on February 8, 1665, close to γ Arietis (‘Mesarthim’) led to the discovery of its duplex nature by Robert Hooke in the course of his observations on the comet. The components, each of the fourth magnitude, and eight seconds apart, are perfectly alike both in light and colour. Meanwhile Huygens had seen θ Orionis—perceived to be quadruple in 1684—as triple in 1656; a Crucis, in the southern hemisphere, was divided by some Jesuit missionaries sent by Louis XIV to Siam in 1685, and α Centauri by Richaud at Pondicherry in 1689; making in all five double stars detected during the seventeenth century.