in regard to the variable star Algol, have revealed that changes of velocity are connected with its changes of lustre. Before minimum the star recedes at the rate of 24½ miles per second, while after minimum the star approaches with a speed of 28½ miles per second (‘Monthly Notices,’ vol. 1. p. 241). _Double Stars and Binary Systems._—Telescopic power will often reveal two stars where but one is seen by the naked eye. Sometimes the juxtaposition of such stars is merely accidental; though they are placed nearly in the same line of sight the conjunction is an optical one only, and no connection apparently subsists between them. In other cases, however, pairs are found which have a physical relation, for one is revolving round the other; and these are termed _binary_ stars. Sir W. Herschel was the first to announce them, from definite observations, in 1802. Of double stars more than 10,000 are now known; many of these are telescopic, but the list includes some fine examples of naked-eye stars. [Illustration: Fig. 62. Double Stars. β Orionis. γ Leonis. α Ursæ Minoris. γ Virginis. δ Cygni. γ Arietis. γ Andromedæ. δ Serpentis. ] Double stars are excellent telescopic tests. A very close pair affords a good criterion as to the defining capacity of an instrument; while a pair more widely separated and of greatly unequal magnitude, like that of α Lyræ, offers a test of the light-grasping power. But in these delicate observations, as, indeed, in all others, the character of the seeing exercises an important and variable influence. A double star that is well shown on one night becomes utterly obliterated on another, owing to the unsteadiness and flaring of the image. On such occasions as the latter one is reminded of the “twitching, twirling, wrinkling, and horrible moulding” of which Sir John Herschel complained, and which unfortunately forms a too common experience of the astronomical observer. A close double, of nearly equal magnitudes, requires a steady night, such as is suitable for planetary details; but a wide double consisting of a bright and a minute star rather needs a very clear sky than the perfection of definition. Certain doubles, such as θ Aurigæ, δ Cygni, and ζ Herculis, are often more easily seen in twilight than on a dark sky; and some experienced observers, conscious of this advantage, have secured excellent measures in daylight. Mr. Gledhill says:—“Such stars as γ Leonis and γ Virginis are best measured before or very soon after sunset” (‘Observatory,’ vol. iii. p. 54). _List of Double Stars._ [Abbreviations in col. 9:—β., Burnham; T., Tarrant; S., Schiaparelli; L., Leavenworth; E., Engelmann; P., Perrotin; Hσ., H. Struve; M., Maw.]