i877*] Physical Changes upon the Moon's Surface. 23 
With these means a series of several thousand observations, 
extending over a period of nearly two years, was made, and 
not only of Plato, but of similarly placed or coloured forma- 
tions. Nothing new, however, resulted ; the interior of 
Plato went through the same course of darkening when 
observed through small fields of view as before, and the 
other regions exhibited their usual course of variations. 
The observation of Mr. ProCtor was not therefore con- 
firmed. 
For the purpose of still more decisively settling this point, 
a piece of apparatus was constructed whereby — through the 
agency of shifting slides of blackened metal, with different 
sized apertures — any two or three different small regions of 
the moon could be seen isolated in the same field, all other 
portions of the surface being cut out. Moreover, by illu- 
minating the upper side of the dark shutters the great con- 
trast between the bright moon and a dark field could be 
subdued. In any case, however, as each of the two or three 
portions were seen at once, under exactly the same condi- 
tions, the effedt of contrast was removed. By this means, 
from time to time, the floor of Plato was compared with 
other small portions of the lunar surface, so that any change 
would be at once detectable, and a numerous series of ob- 
servations made. Under these conditions the darkening in 
the floor of Plato was as marked as ever, though all the 
effects of contrast with the surrounding bright highlands 
were entirely removed, for these bright highlands were shut 
out of view ; and as both portions of the moon were thus 
placed under exactly similar conditions were they of the 
same brightness they should have so appeared when thus 
direCtly compared. By this means the reality of the dark- 
ening of the floor of Plato was incontestibly established, and 
shown to remain unaffected when the effects of contrast are 
thus eliminated.* 
The explanation advanced by Mr. ProCtor to account for 
* Since this was written a statement has appeared in a letter to an English 
periodical, from Mr. Prodtor, to the effedt that, whilst somewhere in America, 
some American gentleman — neither whose address nor name is mentioned — once 
told him that he thought no real darkening in the floor of Plato took place. 
This must be taken quantum valeat, and is only mentioned so that all published 
information on this subjedt may be referred to. A reviewer of my work on the 
Moon, in a very curt and gauche manner, contradidts the above statement, without 
giving any grounds for his behaviour. It is to be presumed he drew on himself for 
the numerous series of observations necessary to authorise a point blank denial 
of a result established by a long series of elaborate observations made with 
powerful instruments. Until these are published, however, — and nothing is 
known of them, — no more need be said of such a contradidtion. 
