50 Condition of the Moon's Surface. [January, 



appear less bright under a full than under an oblique illu- 

 mination, — using the words "full" and "oblique" with 

 reference to the general level of the surface. But the radia- 

 tions occupy arcs of such enormous length upon the moon's 

 surface, that the actual illumination of different parts of the 

 radiations varies greatly, and of course there is a like 

 variation in the illumination of different parts of the regions 

 adjacent. 



It is natural, under these circumstances, to inquire how 

 far it is probable (i) that real processes of change take place 

 month by month on the moon's surface, and (2) that it is to 

 these processes that we owe the greater or lesser distinctness 

 with which certain features present themselves. 



It is known that Dr. De la Rue was led, by his photo- 

 graphic researches into the moon's condition (for we may 

 fairly thus describe his experience in lunar photography), to 

 the conclusion that processes resembling vegetation take 

 place on the moon, the period during which the vegetation 

 passes through its series of changes being a lunar month. 

 He was particularly struck by the circumstance that por- 

 tions of the moon which seem equally bright optically are 

 by no means equally bright chemically. " Hence," he says, 

 " the light and shade in the photograph do not correspond 

 with the light and shade in the picture ; and therefore the 

 photograph frequently renders visible details which escape 

 optically. Those portions of the moon near the dark limb 

 are copied photographically with great difficulty, and it fre- 

 quently requires an exposure five or six times as long to 

 bring out those portions illumined by a very oblique ray, as 

 others apparently not more bright when more favourably 

 illuminated. The high ground in the neighbourhood of the 

 southern portion of the moon is more easily copied than the 

 low ground, usually called seas, and I have ventured to 

 suggest that the moon may have an atmosphere of great 

 density, but of small extent ; and this idea has, I imagine, 

 received some confirmation from a recent observation of 

 Father Secchi's, of the lunar surface polarising light more 

 in the great lowlands and in the bottoms of the craters, 

 apd not appreciably on the summits of the mountain- 

 ridges." 



It is extremely important to notice that photography 

 shows the light near the terminator to be less bright than 

 it appears to the eye. It may be, of course, that the dis- 

 tinction resides mainly or entirely between the photographic 

 power and the luminosity of these portions ; there may, for 

 example, be an excess of yellow light and a deficiency of 



