i 877 *J Physical Changes upon the Moon's Surface . 19 
ance, 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 grey in tint, whilst the surrounding highlands 
are a bright white in colour, tinted with grey ; the appear- 
ance they retain until the thirteenth day after sunrise 
growing a little, though not very much, brighter towards 
full moon. Two days later the floor of Plato has become a 
dark grey; at full moon it is deep steel-grey; and about 
two days after full reaches its darkest tint, a very deep steel- 
grey, almost approaching a black colour. 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 of observations, made 
in the years 1869, 1870, and 18.71, by six or seven indepen- 
dent observers. The character of this change, however, has 
yet to be examined to ascertain whether it is due to a real 
alteration in the tint of the surface, or is merely an optical 
variation due to either the physiological effe< 5 t of contrast 
or to a peculiar conformation of the surface, or to some 
cause of similar nature. 
It is not impossible to imagine a peculiar conformation of 
the surface which would cause it to undergo changes some- 
what resembling those observed in the case of Plato, and in 
facT many such analogous explanations might be devised. 
It, however, must be borne in mind that, though this is 
possible, it involves what is extremely improbable, because 
none of these peculiar conformations can be considered in 
any manner likely. Thus, by supposing the surface of Plato 
to be covered by a number of conical elevations, each of a 
particular shape and height, and distributed in a particular 
manner, and with a systematic relation to every other such 
conical hill, the broad features of the phenomenon might be 
accounted for. This is invoking, however, the aid of a most 
fanciful arrangement, and the probability of any such expla- 
nation is too small to be seriously urged for consideration. 
