154 CARNEGIE INSTITUTION 



a wedge photometer, was published in 1885. In 1886 Miiller and 

 Kerapf at Potsdam entered upon their important investigations in 

 stellar photometr}\ Their first work involved the precise measure- 

 ment of the light of all northern stars in the Durchmusterung of the 

 magnitude 7.5 and brighter. A Zollner's photometer, embodying 

 many improvements, was employed. The first portion of this valu- 

 able work, which was published in 1894, contained measures of 3,522 

 stars between declinations 0° and + 20°. The second portion con- 

 tained measures of 4,416 stars between declinations + 20° and + 40°, 

 and was published in 1899. Meanwhile, at the Harvard Observatory, 

 a new and larger meridian photometer had been constructed, with 

 which more than a million photometric settings have been made. 

 All stars of the magnitude 7.5 and brighter in the Diirchmustenmg 

 and north of declination — 40° have been measured, and the stars 

 of the Harvard Photometry remeasured ; also about 17,000 stars, 

 mainly of the eighth and ninth magnitudes, arranged in zones at 

 intervals of 5° in declination. All of this work has been extended 

 to the South Pole by the establishment of a station in the southern 

 hemisphere at Arequipa, Peru. The number of faint stars is so 

 enormous that it does not seem advisable to attempt to measure them 

 at all. The observations of Herschel show that slow changes of long 

 period occur rarely, if at all, among the brighter stars. It is doubt- 

 ful, therefore, if the meridian photometer need be kept actively at 

 work, except for the purposes named below in connection with vari- 

 able stars, and for measuring other standards. Of course, at inter- 

 vals the brighter stars should be measured, and any tests of system- 

 atic errors affecting the scale established with this instrument will 

 always be of value. 



For measuring the faint stars much remains to be done. At the 

 Harvard Observatory a 12-inch telescope has been mounted horizon- 

 tally and a photometer attached to it, so that stars as faint as the 

 thirteenth magnitude can be measured. The work is made purely 

 differential by always measuring certain stars already measured with 

 the meridian photometer. About 250,000 settings have been made 

 with this instrument during the last four years. The photometric 

 scale has been extended to magnitude 10.5 by measuring a series of 

 zones of Durchmusterung stars, 10' wide and 10° apart in declina- 

 tion. The photometric scale is now being extended to the faintest 

 stars visible in the largest telescopes by the cooperation of the Yerkes, 

 Lick, McCormick, and Harvard Observatories. By a small appro- 

 priation from the Rumford fund and the friendly cooperation of the 



