20 
Physical Changes upon the Moon's Surface. [January, 
Mr. Proctor has advanced, however, a very ingeniously- 
contrived hypothesis to account for the phenomena observed, 
and which he regards as being probably the effects of con- 
trast, and this view he has backed by some sound arguments. 
Mr. Proddor would seek the explanation of the variation in 
the floor of Plato in the same source as that of the well- 
known phenomena of a dark body looking still darker when 
on a brighter background. Thus at sunrise the floor of Plato 
is thrown against a dark background, due to the sombre, 
barely illuminated, surrounding regions, whilst at full moon 
it has for a background the brilliantly illuminated sur- 
rounding highlands, and should look much darker. Thus 
(“ Quarterly Journal of Science,” vol. xl., p. 504, Odd., 1873) 
Mr. Proddor says — “ Setting aside other possible explana- 
tions . . . there is the effedd of contrast to be considered. 
This I believe, from my own observations, to afford the true 
explanation of the observed phenomena. Plato lies on 
lunar highlands, which shine very brilliantly under high 
solar illumination. Towards the Mare Imbrium a compa- 
ratively narrow ridge separates the floor from that region. 
Now when the terminator has just passed beyond Plato, 
the surrounding wall is not nearly so bright as at the time 
of full moon ; the black shadow of its western ridge occu- 
pies the western side of the floor; and the eye, in estimating 
the tint of Plato, is neither oppressed with the glare of 
general light on the one hand, nor forced to compare the 
tint of the floor direddly and solely with a much brighter 
surface ; if the comparison is made on the east, with an 
illuminated wall, it is made on the west with a perfectly 
black shadow-streak. Similar remarks apply to the time 
when the terminator is about to pass away from Plato. 
But at the time of full moon the highlands around Plato are 
very brilliantly illuminated. The glare necessarily makes 
Plato itself look relatively dark, notwithstanding the fadd 
that the floor is also much more brilliantly illuminated ; for 
it is a recognised fadd, that surfaces of unequal light-re- 
fledding capacity appear to differ more in brightness under 
a high illumination than when they are only faintly illumi- 
nated. We know, in fadd, that a surface which is only dark 
looks almost or perfedlly black when itself and a brighter 
background are under strong illumination.” Mr. Proddor 
proceeds to illustrate this by reference to the fadt that occa- 
sionally some of Jupiter’s satellites look bright when seen 
against the sky, and yet dark when seen against the bright 
surface of the planet when in transit, and then continues — 
“ Such an observation as this appears to me to be decisive 
