VARIABLE STARS OF LONG PERTOD. 



135 



magnitudes. Besicles this, ail stars of the ninth magnitude and brighter, 

 in zones 20' wide, and at intervais of 5° in declination, liave been mea- 

 sured according to the same System. We have thns about forty thou- 

 sand stars scattered in ail parts of the sky, whose light is knôwn. Tins 

 work is now being extended to the fainfer stars. With the polarizing 

 photometer attached to the 15-inch Equatorial, two stars can be compa- 

 red photometrically, even if they are as faint as the thirteenth magnitude, 

 provided that they are within half a degree of each other. During the 

 last three years, the writer lias made nearly two liundred thousand pho- 

 tometric comparisons with a télescope of 30 cm. aperture, mounted hori- 

 zontally. Stars as faint as the thirteenth magnitude can be compared 

 with this instrument when within forty minutes of the meridian. The 

 scale of stellar magnitudes in use for the brighter stars may thns be 

 extended to the thirteenth magnitude. This scale will be still further 

 extended by the coopération, which lias been effected by the Yerkes, 

 Lick, Me Cormick, and Harvard Observatories. In this work similar 

 photometers are used on télescopes having apertures of 100, 90, 65, and 

 38 cm. It is expected that in this way, the scale of magnitudes will be 

 extended to the faintest stars visible in the largest télescopes as yet 

 constructed, and that we shall have accurate standards of magnitudes 

 distributed ever the whole sky for stars as faint as the sixteenth, or even 

 the seventeenth magnitude. 



Having thus secured a uuiform and consistent standard scale, the 

 next problem is to reduce the observations of variables to it. The origi- 

 nal observations of the great work of Argelander on variable stars, 

 from the years 1838 to 1867, were published in 1869. As thèse obser- 

 vations were not reduced, but little use could be made of them. A réduc- 

 tion to the photometric scale lias recently been published in Volume 

 XXXIII of the Harvard Aimais. Similar réductions of the observa- 

 tions of Schmidt, ancl of the early observations of Schonfeld, appear 

 in the same volume. Yalentiner has recently published the much more 

 exteiibive, later work of Schonfeld, and it is hoped that the results 

 may be similarly reduced to the photometric scale. The observations 

 made at Harvard, ancl some of those made elsewhere, of sixteen cir- 

 cumpolar variables, from 1889 to 1899, have been reduced to the pho- 

 tometric scale ancl published in full in the Aimais, Yol. XXXYII, 

 Part I. Similar observations of 56 other variable stars of long period are 

 now inthehands of the printer, and will form Part II of the same volume. 



