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 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 extend over a quarter of a century and includes 768 eclipses. Instead of observing 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 components 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 compared. 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 comparison 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 average 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 photometer. 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 reversion. 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 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 magnitude and brighter, in zones twenty minutes wide, and at intervals of
