sociated with orbital revolution. Belopolsky, and later Campbell and Wright have investigated the velocity-curve of η Aquilæ. From their investigations it appears that this variable is a binary, whose period of revolution is of the same duration as the period of the light-changes. The determination of the velocity-curve is accomplished by the use of a slit-spectroscope, which gives a comparison spectrum of some known clement, which is also present in the spectrum of the star. Since the velocity-curve and light-curve are synchronous it might be suspected that the light variation was caused by an eclipse of the star by a relatively dark companion. This can not be true, however, in the case of η Aquilæ, for various reasons. In the first place, the light-curve is not that of an eclipsing star. An eclipse must occur when two stars are both in the line of sight, at which time the apparent motion would be small or zero. As a fact, the minimum of the star does not occur at such a time. The light maximum occurs noticeably later, and the minimum noticeably earlier than the periastron of the star. These facts seem not inconsistent with the theory that the variations in lights are caused by the close approach of the components of a double star moving in elliptical orbits, the outburst of light resulting from some tidal disturbance incited by the enormously increased mutual attractions of the two bodies. An objection to this explanation is that under these circumstances the outburst would probably manifest itself by the presence of bright lines in the spectrum at maximum, as is the case with long-period variables. Small evidence exists that this is true. Another difficulty is found by a comparison of the curves of velocity and light, as determined by Wright and Schur. The former is a smooth curve, while the latter has a secondary maximum. That this may be due in part to an error in the form of the light-curve as given by Schur, seems not impossible, if we compare it with the light-curve of the same star as determined by Pickering with a polarizing photometer. The latter curve shows merely an indication of a secondary maximum. It may be true, of course, that the secondary maximum is sometimes present, at other times absent. That the relation between the curves of motion and light may be most intimate, in some cases at least, is beautifully shown by the variable ω Sagittarii. The velocity-curve of this star was determined by Dr. Curtiss, of the Lick Observatory. As pointed out by him, the velocity-curve, and the light-curve determined by Professor Pickering, show a close resemblance even in the details, which proves conclusively that both phenomena are associated with the same underlying causes. Incidentally, a striking proof is furnished of the accuracy of these two widely separated investigations, thus critically compared. These curves are shown in Fig. 5.
β Lyræ represents a somewhat different variety of the short-period variable. This star has been studied for more than a century and still remains something of a mystery. The spectrum is complex, the lines
