HISTORY OF PHYSICAL ASTRONOMY.
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1606 noted it as a star of the third magnitude, with a corresponding ob-servation of his own, he determined the period more accurately to beS <1 20 h 48 ra 66 s *.
Another interesting star, the discovery of whose variability is due toGoodricke, is & Lyra. It was first recognised by him as subject to changesof brightness in the year 1784. He was originally induced to fix theperiod of variation at 6 d 9 h ; but after continuing his observations forsome time, he found that, although the course of the phases very nearlyagreed with this period, an exact compensation of brightness did not takeplace in less than 13 d 19 h , there being in reality two maxima and twominima. When it attains its maximum brightness, it is equal to a star ofthe third magnitude. At one of its minima, it appears between thefourth and fifth magnitude, and at the other between the third andfourth f. These interesting particulars have been fully confirmed by therecent researches of M. Argelander.
It may be remarked respecting variable stars, that in passing throughtheir successive phases, they are subject to sensible irregularities, whichhave not hitherto been reduced to fixed laws. In general theydo not always attain the same maximum brightness, their fluc-tuations being in some cases very considerable. Thus, according toArgelander, the variable star in Corona Borealis , which Pigott dis-covered in 1795, exhibits on some occasions such feeble changes ofbrightness, that it is almost impossible to distinguish the maxima fromthe minima by the naked eye; but after it has completed several of itscycles in this manner, its fluctuations all at once become so considerable,that in some instances it totally disappears. It has been found,moreover, that the light of variable stars does not increase and di-minish symmetrically-on each side of the maximum, nor are the succes-sive intervals between the maxima exactly equal to each other.
Several persons have endeavoured to account for the phenomena ofvariable stars, but no satisfactory view of the subject has yet beenarrived at by any enquirer. Bouillaud sought to account for the variationsof o Ceti, by attributing to the star a rotation round a fixed axis,and supposing the greater part of its surface to be obscure. Accordingto Maupertius, some stars, by whirling rapidly round their axes, becameflattened to such an extent as to resemble millstones : they were, more-over, liable to periodic perturbations from the action of opaque bodiesrevolving round them. Hence, when one of such stars presented itsflattened surface to the earth, it shone with its maximum brightness,and when seen edgewise, its lustre was manifestly a minimum. Good-ricke suggested that the variations of Algol might perhaps arise from theperiodic transit of an opaque planet revolving round the star. Pigottassimilated the phenomena of variable stars to the solar spots, and thisview of the subject is beyond doubt the most satisfactory that has yetbeen advanced.
The Photometry of the stars, although forming one of the most im-portant subjects of Stellar Astronomy, has hitherto remained in a veryimperfect condition. The usual mode of designating the brightness ofthe stars by arranging them according to different magnitudes, is bothvague in theory and contradictory in practice. It is vague, inasmuch asthe place of a star in the scale of magnitudes conveys no definite idea of
Phil. Trans., 1784, p. 289.
f Phil. Trans., 1785, p. 153, et seq.