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).
[Abbreviations in col. 9:—β., Burnham; T., Tarrant; S., Schiaparelli; L., Leavenworth; E., Engelmann; P., Perrotin; Hσ., H. Struve; M., Maw.]