1877 *] Physical Changes upon the Moon's Surface. zt 
against mere eye-estimations, showing that absolutely no 
reliance can be placed on them unless some contrivance is 
employed to destroy the effeCt of contrast. As it is conceded 
that none of the observed darkenings of the floor of Plato 
have been other than eye-estimations, no further discussion 
seems needed, or can in my opinion be legitimately given to 
the subject. I may mention, however, that having, though 
as yet in an imperfeCt manner, studied Plato with a much 
reduced field, so that I could eliminate to some degree the 
effedts of contrast, I have not found that the floor grows 
relatively darker towards the time of full moon.” 
This most ingenious explanation devised by Mr. ProCtor to 
account for the phenomena presented by the floor of Plato, 
and by assuming the correctness of which he is enabled to 
draw the very strong conclusions just quoted, is not an ex- 
planation which will be for one moment admitted by astro- 
nomers who are familiar with the phenomena presented by 
the various lunar formations, for they at once perceive 
numerous and apparently fatal objections to it, when applied 
as Mr. ProCtor proposes to do ; its very nature, in fadt, 
suggests the idea that its able author overlooked the relation- 
ship in which Plato and the anomalous darkening of its 
floor stood to other similar formations with the normal be- 
haviour of their interiors, for his hypothesis supposes nothing 
exceptional in the constitution of the walled plain Plato. 
If, then, the great darkening observed to occur in the tint of 
the interior of Plato is merely apparent, and only what must 
occur when a darkish walled plain is surrounded by a bright 
background, or rather bright environs,— and this is all 
Mr. ProCtor ascribes to it,— it must be a perfectly normal 
occurrence, and the same must take place in every similarly 
placed formation, unless that has something anomalous 
about it to prevent this taking place. But all selenographers 
could instance a number of such walled plains where no 
such darkening occurs. Are we, then, to assume that all 
these possess anomalously constituted interiors, and that only 
Plato, of all the lunar formations, exhibits the normal phe- 
nomena. This is, of course, entirely inadmissible, and 
selenographers are thoroughly aware that the effects of con- 
trast alluded to by Mr. ProCtor are entirely incapable of 
bringing about such an immense darkening in tint as is 
apparent in the case of the floor of Plato. 
As an instance illustrating this, the behaviour of the 
walled plain named Archimedes maybe considered in com 
neCtion with that of Plato. This walled plain lies not very 
far to the south of Plato, and is, if slightly smaller, of the 
