when the thickness of it they have power to traverse furnishes a
criterion of their intensity. Professor Pickering's "meridian
photometer," on the other hand, is based upon Zoellner's principle of
equalization effected by a polarising apparatus. After all, however, as
Professor Pritchard observed, "the eye is the real photometer," and its
judgment can only be valid over a limited range.[1607] Absolute
uniformity, then, in estimates made by various means, under varying
conditions, and by different observers, is not to be looked for; and it
is satisfactory to find substantial agreement attainable and attained.
Only in an insignificant fraction of the stars common to the Harvard and
Oxford catalogues discordances are found exceeding one-third of a
magnitude; a large proportion (71 per cent.) agree within one-fourth, a
considerable minority (31 per cent.) within one-tenth of a
magnitude.[1608] The Harvard photometry was extended, on the same scale,
to the opposite pole in a catalogue of the magnitudes of 7,922 southern
stars,[1609] founded on Professor Bailey's observations in Peru,
1889-91. Measurements still more comprehensive were subsequently
executed at the primary establishment. With a meridian photometer of
augmented power, the surprising number of 473,216 settings were made
during the years 1891-98, nearly all by the indefatigable director
himself, and they afforded materials for a "Photometric Durchmusterung,"
published in 1901, including all stars to 7.5 magnitude north of
declination -40 deg.[1610] A photometric zone, 20 deg. wide, has for some
time been in course of observation at Potsdam by MM. Mueller and Kempf.
The instrument employed by them is constructed on the polarising
principle as adapted by Zoellner.

Photographic photometry has meanwhile risen to an importance if anything
exceeding that of visual photometry. For the usefulness of the great
international star-chart now being prepared would be gravely compromised
by systematic mistakes regarding the magnitudes of the stars registered
upon it. No entirely trustworthy means of determining them have,
however, yet been found. There is no certainty as to the relative times
of exposure needed to get images of stars representative of successive
photometric ranks. All that can be done is to measure the proportionate
diameters of such images, and to infer, by the application of a law
learned from experience, the varied intensities of light to which they
corresp