To illustrate more clearly the difficulty of identification of the same region under different illuminations, if the reader will turn, for instance, to region 8, he will find that, in the southern portion especially, under phase C, it is impossible to identify any particular crater recognised in phase A by a direct comparison of these two plates. A use of the intermediate plate B, however, will enable him, after a little study, to find any desired formation.
The north polar regions are best shown on Plate 9B and the south polar regions on Plate 8C. The highest peak upon the Moon, according to Neison, attaining an altitude of about 30,000 feet, is situated in the Leibnitz Mountains near the South Pole, and is indicated upon the maps by a small cross. The alterations in appearance that similar craters undergo with a given difference in phase are sometimes quite tmlike. Thus, on Plate11A the craters Copernicus [2.5, 6.9] and Eratosthenes [1.4, 6.2] are very much alike in appearance. But while on Plate 11C Eratosthenes [0.6, 6.5] has grown darker, Copernicus [1.6, 7.1], on the other hand, has grown lighter, and the resemblance has ceased to exist. On iiA the craters Reinhold [2.9, 7.7] and Landsberg [3.4, 8.2] are quite conspicuous, while the small bright craterlet [2.4, 8.0] southwest of the former would scarcely excite notice. On iiC the large craters have almost disappeared, while the smaller one is now a conspicuous object. Similarly on 11A Pytheas [2.4, 5.4] and Lambert [2.4, 4.8] are not unlike, but on 11C Lambert is almost invisible, while Pytheas is a brilliant object. It is obvious that changes of this character can only be adequately represented by a photographic atlas, and by one representing the same region at several phases taken at very different distances from the terminator.
One possible use to which the atlas may be put is to suggest research. Thus the changes occurring in and about Eratosthenes were discovered by observations in part suggested by a study of the atlas which was at that time in process of construction. We must be careful, however, not to put too much confidence in results obtained from a mere comparison of photographs, as slight changes in exposure and development will sometimes produce results that are very misleading. The only safe course to follow is to confirm all suspected changes by a careful visual study of the formation at different ages of the Moon, and if near the limb the changes should be studied also under different librations.
As an instance of an apparent photographic change, we may, on Plate 5A, take the snow in the crater Plinius [1.9, 6.6]. If we turn now to Plate 5B we shall find that
