STUDY OF THE VARIABLE STABS 183 



sociated with orbital revolution. Belopolsky, and later Campbell and 

 Wright have investigated the velocity-curve of ^ Aquilae. 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 ve- 

 locity-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 -q Aquila?, 

 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. A> 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 pres- 

 ence of bright lines in the spectrum at maximum, as is the case with 

 long-period variables. Small evidence exists that this is true. An- 

 other 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 to 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. 



(3 Lyra3 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 



