THE LIGHT OF THE STABS. 49 



determined, as in the Pritchard, and nearly all other photometers. 

 The principal objections to this instrument are, first, that stars can 

 not be compared unless they are near together, and, secondly, that faint 

 stars can not be measured, since one half of the light is lost by polari- 

 zation. The principal uses so far made of this form of photometer are 

 in comparing the components of double stars, and in a long series of 

 observations of the eclipses of Jupiter's satellites, which now extend 

 over a quarter of a century and includes 768 eclipses. Instead of ob- 

 serving the time of disappearance, a series of measurements is made, 

 which gives a light curve for each eclipse. Much important work 

 might yet be done with this form of photometer, in measuring the com- 

 ponents of doubles and of clusters, and in determining the light curves 

 of variables which have a moderately bright star near them. 



An important improvement was made in this form of photometer 

 in 1892 by which stars as much as half a degree apart could be com- 

 pared. The cones of light of two such stars are brought together by 

 achromatic prisms, so that they can be compared as in the preceding 

 instrument. As there is no part of the sky in which a suitable com- 

 parison star can not be found within this distance, any star may be 

 measured with this instrument. In the hands of Professor Wendell 

 this photometer has given results of remarkable precision. The aver- 

 age deviation of the result of a set of sixteen settings is about three 

 hundredths of a magnitude. Light curves of variables can therefore 

 be determined with great precision, and suspected variables can be 

 divided into those that are certainly variable, and those whose changes 

 are probably less than a tenth of a magnitude. 



Another change in this instrument produced the meridian pho- 

 tometer. Instead of using the two cones from one object glass, two 

 object glasses were used, mirrors being placed in front of each. In 

 this way stars however distant can be compared. In theory this instru- 

 ment leaves but little to be desired. Almost every source of error that 

 can be suggested can be eliminated by proper reversion. As con- 

 structed, the telescope is placed horizontally, pointing east or west. 

 One mirror reflects a star near the pole into the field, the other, a star 

 upon the meridian. A slight motion of the mirror permits stars to 

 be observed for several minutes before or after culmination. The first 

 meridian photometer had objectives of only two inches aperture. With 

 this instrument 94,476 measures were made of 4,260 stars during the 

 years 1879 to 1882. All stars were included of the sixth magnitude 

 and brighter, and north of declination — 30°. 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 magni- 

 tude and brighter, in zones twenty minutes wide, and at intervals of 



VOL. LXVI. 4. 



