[Nichol found that the rays from Kepler cut through rays from Copernicus and Aristarchus, while rays from the latter cut through rays from the former.  He therefore inferred that their relative ages stand in the order,—­Copernicus, Aristarchus, Kepler.]

As no branch of selenography has been more neglected than the observation of these interesting but enigmatical features, one may hope that, in spite of the exacting conditions as to situation and instrumental requirements necessary for their successful scrutiny, the fairly equipped amateur in this less favoured country will not be deterred from attempting to clear up some of the doubts and difficulties which at present exist as to their actual nature.

THE MOON’S ALBEDO, SURFACE BRIGHTNESS, &c.—­Sir John Herschel maintained that “the actual illumination of the lunar surface is not much superior to that of weathered sandstone rock in full sunshine.”  “I have,” he says, “frequently compared the moon setting behind the grey perpendicular facade of the Table Mountain, illuminated by the sun just risen in the opposite quarter of the horizon, when it has been scarcely distinguishable in brightness from the rock in contact with it.  The sun and moon being at nearly equal altitudes, and the atmosphere perfectly free from 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.