24 Physical Changes upon the Moon's Surface. [January, 
the darkening of the floor of Plato is not therefore adequate 
for its purpose. Unquestionably it may have some effedt, 
but whatever its amount may be it is too small to be sensible. 
Some small variations in the amount of the darkening of 
Plato, when observed by the various methods detailed, were 
also found to exist, but they were relatively very small in 
amount. These differences were, however, no more than 
were to be expedled. 
What, then, is the cause of this singular darkening in the 
floor of Plato ? What reason can exist why that, as the 
solar altitude increases, the interior of Plato — -unlike any 
other similar formation upon the surface of the moon — - 
should alter in colour and brightness from a cold yellow-grey 
to a deep steel-grey, or almost black tint ? It arises evi- 
dently, from what has been stated, from the anomalous con- 
ditions prevailing upon the surface of the interior of the 
wall plain. It is extremely improbable that*these anomalous 
conditions should be an exceptional and fanciful configuration 
of the surface, and it is more natural to suppose that they 
exist in reference either to the condition or constitution of 
the surface. But however successfully the subject may 
have been pursued so far, here all further progress in eluci- 
dating this question is suddenly arrested, — no materiel exist 
for a further investigation. The observations now required 
are not those of simple observation with the usual astrono- 
mical appliances ; what is wanted is special observations 
with special appliances, and these are at present not to be 
obtained. And, in fadt, the great difficulties in the way of 
making these observations, and the powerful and costly in- 
struments, and the experience required, effectually debar all 
except a few favoured astronomers from pursuing these 
observations. Thus, as in so many selenographical problems, 
patient observation establishes the existence of certain phe- 
nomena, but the elucidation of the meaning of the pheno- 
mena established is checked for want of special observations 
that are never made. For these selenographers have to 
appeal to those astronomers devoted to what has been 
termed astronomical physics, but they are too much engaged 
on, to them, more fascinating subjects to be able to assist 
selenographers. 
Although, therefore, the means do not exist for saying, in 
one manner or another, this change results from this cause 
or from that cause, it is not fruitless to enquire what their 
studies have led selenographers to believe to be the cause of 
this change. The opinion, then, that appears to agree with 
the views of the most experienced selenographers who have 
