powerful telescopes are indispensable. Unfortunately, it is a difficult matter to study, for on only one or two days in the year is Messier likely to be found in a proper condition to be observed.
Other instances of change in the lunar surface have been detected by selenographers, but the evidence on which they rest is not so overpowering as is needed to induce their acceptance.
Periodical variations in the color and brightness of lunar regions, such as would result from processes of vegetation, were first noticed by Beer and Mädler, but they regarded the absence of masses of water upon the moon as a fatal objection to this explanation. The variation in the floor of Plato is one of the most interesting of these changes. Plato is a circular plain 60 miles in diameter, surrounded by a belt of highlands from 3,000 to 3,500 feet in height. These highlands at sunrise are a pale yellowish gray, gradually changing to grayish white. At sunrise the interior of Plato appears of a cold gray, but, as the sun rises higher above the horizon of Plato, and the solar rays fall more perpendicularly on this region, the whole surface grows rapidly brighter, until, about two days after sunrise, the interior of the formation attains its brightest tint. It is then a cold, light yellow gray, often approaching a pale yellow, in fact, and brighter than the surface of the Mare Imbrium on the north, while the surrounding highlands are a bright grayish white, tinted here and there with gray. Judging from what occurs in any of the numerous other formations resembling Plato, this may be considered the normal tint, inasmuch as those other formations which present exactly the same phenomena up to this time, and which, under similar conditions, present exactly the same appearance, retain this tint unaltered until near sunset. After, however, the second day, the floor of Plato commences to undergo a most extraordinary and anomalous change, which renders it unique on the moon, for, instead of growing lighter, the interior commences to become darker. Four days after sunrise it is materially darker than the northern Mare, and a cold gray in tint, while the surrounding highlands are a bright white in color, tinted with gray; the appearance they retain until the thirteenth day after sunrise, growing a little, though not very much, brighter toward full moon. Two days later the floor of Plato has become a dark gray; at full moon it is deep steel-gray; and, about two days after full, reaches its darkest tint, a very deep steel-gray, almost approaching a black color. Under these conditions, it is one of the very darkest portions of the entire lunar surface, though, seven days prior, it was one of the lightest portions of the surface of its kind. After this, it gradually lightens in tint, but much slower, and never reaches so light a tint.
This extraordinary periodical change in the tint of the floor of Plato has hitherto received no explanation, but its existence has been put beyond the pale of doubt, and Mr. Birt has, at the instance of a British Association Committee, carefully discussed a numerous series
