THE LIGHT OF THE STARS 377 



affected equally by passing clouds, so that this photometer may be 

 used whenever the stars are visible, and at times when other photo- 

 metric work is impossible. The diminution in light also follows a 

 simple geometrical law, and is readily computed with great accuracy. 

 There is no unknown constant to be determined, as in the Pritchard, 

 and nearly all other photometers. The principal objections to this 

 instrument are, first, that stars cannot be compared unless they are 

 near together, and, secondly, that faint stars cannot be measured, 

 since one half of the light is lost by polarization. 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 extends over a quarter of a century 

 and includes 768 eclipses. Instead of observing the time of disappear- 

 ance, 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 components of doubles and of 

 clusters, and 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 preced- 

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

 comparison star cannot 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 average 

 deviation of the result of a set of sixteen settings is about thres 

 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 photo- 

 meter. 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 

 instrument leaves but little to be desired. Almost every source of 

 error that can be suggested can be eliminated by proper reversions. 

 As constructed, 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 per- 

 mits stars to be observed for several minutes before or after culmin- 

 ation. The first meridian photometer had objectives of only two 

 inches aperture. With this instrument, 94,476 measures were made 

 of 4260 stars, during the years 1879 to 1882. All stars were included 

 of the sixth magnitude and brighter, and north of declination 30. 



