378 ASTRONOMY 



The second instrument had objectives of four inches aperture, and 

 permitted stars as faint as the tenth magnitude to be measured. With 

 this instrument, during the years 1882 to 1888, 267,092 measures 

 were made of 20,982 stars, Including all the catalogue stars and all 

 the stars of the ninth magnitude and brighter, in zones twenty min- 

 utes wide, and at intervals of five degrees, from the north pole to 

 declination 20. In 1889, the instrument was sent to South Amer- 

 ica, where 98,744 measures were made of 7922 southern stars, extend- 

 ing 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 5332 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 photo- 

 meter has been sent to South America. The total number of meas- 

 urements 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 0.08. 



The principal objection to the instrument just described is the 

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

 meter has been devised. A 12-inch telescope has been mounted 

 horizontally, like the meridian photometer, and an artificial star re- 

 flected 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 

 measurement of all the stars in the Bonn Durchmusterung , which are 

 contained 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 magnitudes 

 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 re- 



