A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 53 



of this point may possibly be made by a careful study of sundry parts of the lunar 

 surface where outside of the vulcanoids there are features which may have to be 

 accounted for on the hypothesis of meteoric falls of masses great enough to 

 produce some local melting but not sufficient to create distinct maria. 



AREAS OF VARIABLE HUE. 



Of all the diversities of hue observable in the lunar surface, those which vary 

 from time to time are the most curious and the most baffling to the inquiry. 

 The objects of this class may conveniently be divided into two groups, of which 

 the first should include the irregular patches of light generally capping the flanks 

 and ramparts of the vulcanoids and the cones they enclose, together with the 

 bands of light color which in most instances radiate from vulcanoids or origi- 

 nate near them. It is characteristic of the objects in this group that they are 

 invisible or nearly so when the sun is just rising on them, that they commonly 

 are not noticeable, indeed, until the sun is high, and that they disappear when the 

 illumination becomes again very oblique. The other group contains sundry 

 examples where the fields are lighter colored in low than in high illumination, in 

 this regard reversing the conditions of the first named series. There are no 

 features on the moon's surface which have been the subject of more inquiry, 

 though mostly of a discursive kind, than the first-named group of colored areas. 

 The hypotheses and speculations concerning them have been numerous, but have 

 led to no accepted judgment concerning them. It appears to me that the best 

 way to approach the problem they afford is that indicated below. 



First let us note that by far the greater area of the fields, which suddenly 

 become very white as the lunar day advances, lies on the higher part of the vulca- 

 noids, on their slopes and the summits of their enclosed cones. It is evident, 

 therefore, that the whiteness is most likely due to some quality of the surface 

 imparted by the vulcanoid action to which these regions have been exposed, a 

 quality which is developed only under a rather high sun. Under these conditions 

 the measure of whiteness is roughly proportional to the approach of the illumina- 

 tion to verticality, perhaps not absolutely so, for it is held by most observers that 

 probably the brightest point on the moon's surface is the central peak of Aristar- 

 chus which lies about twenty-three degrees south of the equator. I am inclined, 

 however, to believe that the apparent extreme brightness of this object is due to 

 the contrast afforded by the dusky fields of the mare in which it lies, and that the 

 fields of extremest lucency are all nearer the central part of the moon. 



That the brightness of the very shiny parts of the moon, the patches and 

 the rays alike, is not due to any change in their constitution brought about by 

 the action of the sun during the monthly fourteen days of illumination, is proved 

 by the fact that these features distinctly appear on the moon's surface when, in its 

 newest stage, it is receiving a like vertical earth-light. I noted this fact many 

 years ago, though I did not then perceive its full significance. I am now assured 

 that my observations were trustworthy for the reason that negatives of the dark 



