5 o POPULAR SCIENCE MONTHLY. 



five degrees, from the north pole to declination — 20°. In 1889 the 

 instrument was sent to South America, where 98,744 measures were 

 made of 7,922 southern stars, extending the two preceding researches 

 to the South Pole. On the return of the instrument to Cambridge 

 473,216 measures were made of 29,587 stars, including all those of the 

 magnitude 7.5 and brighter north of declination — 30°. This work 

 occupied the years 1891 to 1898. The instrument was again sent to 

 Peru in 1899, and 50,816 measures were made of 5,332 stars, including 

 all those of the seventh magnitude and brighter, south of declination 

 — 30°. The latest research has been the measurement of a series of 

 stars of about the fifth magnitude, one in each of a series of regions 

 ten degrees square. Each of these stars is measured with the greatest 

 care on ten nights. This work has been completed and published for 

 stars north of declination — 30°, 59,428 measures having been made 

 of 839 stars. In this count, numerous other stars have been included. 

 Similar measures are now in progress of the southern stars, this being 

 the third time the meridian photometer has been sent to South America. 

 The total number of measurements exceeds a million, and the number 

 of stars is about sixty thousand. About sixty stars can be identified 

 with care, and each measured four times with this instrument in an 

 hour. The probable error of a set of four settings is =t 0.08. 



The principal objection to the instrument just described is the 

 great loss of light. To measure very faint stars, another type of 

 photometer has been devised. A twelve-inch telescope has been 

 mounted horizontally, like the meridian photometer, and an artificial 

 star reflected into the field. The light of this star is reduced by a 

 wedge of shade glass until it appears equal to the star to be measured. 

 Four hundred thousand measures have been made with this instrument 

 during the last five years. The principal research has been the meas- 

 urement of all the stars in the Bonn Durchmusterung which are con- 

 tained in zones ten minutes wide and at intervals of five degrees, from 

 the north pole to declination — 20°. Large numbers of stars of the 

 tenth and eleventh magnitudes are thus furnished as standards of light. 

 As the light of the object observed is unobstructed, any star however 

 faint, if visible in the telescope, may be measured. Accordingly, many 

 stars of the twelfth and thirteenth magnitude have been selected and 

 measured, thus furnishing faint standards. Sequences of standard 

 stars have been selected from coarse clusters, thus permitting estimates 

 or measures of these bodies to be reduced to a uniform photometric 

 scale. An investigation of great value has been carried out success- 

 fully at the Georgetown College Observatory by the Eev. J. G. Hagen, 

 S.J. All the stars of the thirteenth magnitude and brighter have been 

 catalogued and charted in a series of regions, each one degree square, 

 surrounding variable stars of long period. Besides measuring the po- 

 sitions he has determined the relative brightness of these stars. A 

 sequence has then been selected from each of these regions, and meas- 



