22 Physical Changes upon the Moon's Surface . [January, 
same form, and bordered by mountains of the same height 
and brightness ; whilst tying on the open plain it is environed 
by a surface little inferior in brightness to that around Plato. 
At the period when the anomalous darkening of the floor of 
Plato commences, the interiors of the two formations are 
sensibly of the same tint, a cold light yellow grey, both 
being brighter than the surface of the intervening Mare 
Imbrium. But while the interior of Plato darkens until it is 
almost black, the floor of the similar formation Archimedes 
grows gradually lighter until it is a bright yellowish grey, 
thus following the same course as nearly every other lunar 
formation. If, then, the darkening presented by the floor 
of Plato is, as Mr. Prodtor contends, the natural result of the 
contrast between its surface and its bright environs, why 
does not Archimedes, similarly placed, and with a floor at one 
period of similar darkness, present the same phenomena ? 
The fadt that Archimedes and the many other similarly 
placed formations do not present this darkening, and that 
Plato alone of the many hundred lunar-walled plains does 
darken to this great extent, shows that there is something 
anomalous in the constitution of its interior, which is what 
selenographers contend. It is obvious, therefore, that 
the hypothesis framed by Mr. Prodtor to account for the 
phenomena fails to do so when critically examined. 
In his paper, Mr. Prodtor refers, however, to what he 
considers diredt evidence that the darkening of the floor of 
Plato is a mere effedt of contrast, namely, that when 
examined with a small field of view, so as to shut out the 
greater portion of the surrounding bright environs, the 
interior of the walled plain showed no darkening towards 
full. This observation of Mr. Prodtor’s, if confirmed, would 
have been extremely important, because it would establish 
the fadt of the darkening of the interior of Plato being 
merely the effedt of contrast, and so make the general 
behaviour of the similarly situated walled plains the real 
anomaly. The observation of Mr. Prodtor was confessedly 
imperfedt, but the point required careful examination. For 
this purpose a numerous series of observations, with the 
field of view of the telescope purposely construdted to small 
dimensions, was undertaken, in some the field being reduced 
so small that only a portion even of the interior of Plato 
could be seen at once, so that the entire environs were shut 
out ; in others, the border of the walled plain was included; 
and in others, more or less of the surrounding brighter regions 
in all less different sized fields were employed, a revolving 
diaphragm with apertures of different sizes being employed. 
