cloud or vapour, its effect is alike on both luminaries."
Zollner's elaborate researches on this question are closely in accord
with the above observational result. Though he considers that the
brightest parts of the surface are as white as the whitest objects with
which we are acquainted, yet, taking the reflected light as a whole, he
finds that the moon is more nearly black than white. The most brilliant
object on the surface is the central peak of the ring-plain Aristarchus,
the darkest the floor of Grimaldi, or perhaps a portion of that of the
neighbouring Riccioli. Between these extremes, there is every gradation
of tone. Proctor, discussing this question on the basis of Zollner's
experiments respecting the light reflected by various substances,
concludes that the dark area just mentioned must be notably darker than
the dark grey syenite which figures in his tables, while the floor of
Aristarchus is as white as newly fallen snow.

The estimation of lunar tints in the usual way, by eye observations at
the telescope, involving as it does physiological errors which cannot be
eliminated, is a method far too crude and ambiguous to form the basis of
a scientific scale or for the detection of slight variations. An
instrument on the principle of Dawes' solar eyepiece has been suggested;
this, if used with an invariable and absolute scale of tints, would
remove many difficulties attending these investigations. The scale which
was adopted by Schroter, and which has been used by selenographers up to
the present time, is as follows:--

0 deg. = Black.
1 deg. = Greyish black.
2 deg. = Dark grey.
3 deg. = Medium grey.
4 deg. = Yellowish grey.
5 deg. = Pure light grey.
6 deg. = Light whitish grey.
7 deg. = Greyish white.
8 deg. = Pure white.
9 deg. = Glittering white.
10 deg. = Dazzling white.

The following is a list of lunar objects published in the
_Selenographical Journal_, classed in accordance with this scale:--

0 deg. Black shadows.
1 deg. Darkest portions of the floors of Grimaldi and Riccioli.
1 1/2 deg. Interiors of Boscovich, Billy, and Zupus.
2 deg. Floors of Endymion, Le Monnier, Julius Caesar, Cruger, and
Fourier _a_.
2 1/2 deg. Interiors of Azout, Vitruvius, Pitatus, Hippalus, and Marius.
3 deg. Interiors of Taruntius, Plinius, Theophilus, Parrot,
Flamsteed, and Mercator.
3 1/2 deg. Interiors of Hansen, Archimedes, and Mersenius.
4 deg. Interiors of M