1 8 Physical Changes upon the Moon's Surface. [January, 
most interesting of these changes in tint, because its reality 
and its periodical character have been established in an in- 
disputable manner. Plato is a circular plain, 60 miles in 
diameter, lying depressed beneath the surface of a great 
belt of highlands, which separate two of the greater lunar 
dark plains from one another. The formation lying in 50° N. 
latitude is seen from the earth in perspective, and appears 
to be foreshortened into a marked ellipse. It is bordered by 
a moderately steep slope, from 3000 to 3500 feet in height, 
broken here and there on the west and east by peaks nearly 
as high again. On the south it is separated by this high- 
land border from the dark plain called the Mare Imbrium , 
to the level of which the border falls in a gentle slope, the 
distance between the floor of Plato and the Mare Imbrium 
being about 10 miles. The plain forming the interior of 
Plato is almost level, being broken only by a few very small 
steep crater cones. The colour of the highlands around 
Plato is a pale yellowish grey at sunrise, gradually becoming 
a greyish white as they are more intensely lit up by the 
solar rays. The grey plain on the north of Plato, called 
the Mare Imbrium, is a pure yellow-grey at sunrise, gra- 
dually increasing in brightness until a light yellowish grey 
at full moon. These details are necessary to properly un- 
derstand the change which occurs in the tint of the floor of 
Plato as the lunar day progresses from sunrise to noon, and 
thence to sunset. The details of the changes experienced 
are founded on six years’ observations. 
At sunrise the interior of Plato appears a pure cold grey 
in colour, whilst the surrounding highlands are a yellowish 
grey, and the neighbouring Mare Imbrium a cold yellow-grey, 
on the western portion of the floor being the deep black 
shadow of the border, and on the eastern side the greyish 
white illuminated slope of a lofty peak. 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-grey, often approaching a pale 
yellow in faCt, and brighter than the surface of the Mare 
Imbrium on the north, whilst the surrounding highlands are 
a bright greyish white, tinted here and there with grey. 
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 pre- 
sent exactly the same phenomena up to this time, and which 
under similar conditions present exadtlv the same appear- 
