112 PHOTOGRAPHY OF THE LUNAR SURFACE. 



not be identified. If two such drawings were considered as relating to 

 different epochs, there would have been a great temptation to assume 

 that there had been a profound transformation, while everything is 

 explained by errors of interpretation of a physiological origin, which 

 arise when the attention is concentrated for a long time on objects 

 which are scarcely visible. The situation would be entirely different 

 for the moon if we had negatives taken fifty or sixty years ago com- 

 parable in quality with those obtained at the present time. Under 

 these circumstances there would be no uncertainty as to surface changes 

 of any moment. 



VI. — INDICATIONS FURNISHED BY RECENT PHOTOGRAPHS IN REGARD 

 TO THE ORIGIN OF THE LUNAR CORE. 



Let us now examine what profit science can hope to reap from the 

 study of photographs taken in the last few years. They confirm, on 

 the whole, the correctness of the work of Madler and Schmidt, and 

 show that no phenomenon which has modified in any permanent man- 

 ner the general aspect of the moon has taken place in the last half cen- 

 tury. Concerning local variations, it would be wise not to draw any 

 absolute conclusions and to consider the recent photographs as bea- 

 cons erected for the future. The value of lunar photographs can not 

 fail to increase with time, and they will doubtless permit us to draw 

 definite conclusions within a few years which the sketches of two cen- 

 turies ago would not authorize. Moreover, the photographs are even to 

 furnish us at once with other very important data from another point of 

 view, for giving, as they do, a homogeneous and simultaneous represen- 

 tation of the whole visible disc, they lend themselves very well indeed 

 to a study of the origin of the lunar soil. They possess particular 

 advantages in the recognition of the general alignments in the struc- 

 ture of the soil, the delicate tracings which extend for great distances, 

 such as the so-called cracks and radiating streaks. 



These objects are most difficult to distinguish by direct observations, 

 the attention of the astronomer being in this case strongly concentrated 

 on a restricted portion of the image. Another valuable property of 

 photography is that it brings out in an expressive manner, possibly 

 slightly exaggerated, the differences in the tint of two neighboring 

 regions. Such indications have a great value in the study of the actual 

 physical state of the moon, and, notably, in proving the presence or 

 absence of air and water on its surface. We are now to take up a con- 

 sideration of this question, which is intimately connected with the prob- 

 lem so often suggested of the habitability of the plauets. 



VII. — ON THE EXISTENCE OF "WATER AND AIR ON THE MOON. 



The complete absence of air and of water on the moon would be con- 

 sidered at first sight as an abnormal condition. It is indeed difficult, 

 whatever cosmic theory one adopts, not to regard the moon as a depend- 



