106 PHOTOGRAPHY OF THE LUNAR SURFACE. 



planet, therefore, seems capable of giving us greater information in 

 regard to the question whether the physical forces (the effect of which 

 we observe on our globe) act similarly on othc r celestial bodies. It lends 

 itself, perhaps, better than the earth, on account of exposing to view a 

 whole hemisphere at once, to the study of certain captivating and diffi- 

 cult problems, those for instance concerning the origin and the evolu- 

 tion of satellites in general. Indeed, one can not examine the moon 

 without being led to ask if conditions favorable to the development of 

 vegetation and of life can be found on other celestial bodies beside the 

 earth, a question too often agitated, although always attracting philo- 

 sophical interest. 



The first serious attempts made to attack this problem date to the 

 invention of the telescope. But, although skillful observers have 

 devoted themselves to a study of the moon, it is remarkable that until 

 recently no powerful telescope had ever been applied to a study of our 

 satellite as a whole. Under these conditions the multitude of visible 

 details is so great that an isolated investigator can not undertake to 

 describe or to delineate them. On the other hand, if the task be 

 divided among a number of astronomers, or if the attention be fixed on 

 some particular region, there is the risk in either case of losing the char- 

 acter of unity of the undertaking, and in addition the physiological or 

 accidental errors of observation would impart a provoking uncertainty 

 to any conclusions which might be thus deduced. 



II. — ADVANTAGES OFFERED IN THIS RESPECT BY PHOTOGRAPHY. 



The invention of photography opened an avenue of escape from these 

 difficulties. However, many years have been necessary for astronomers 

 to put to application all the advantages offered to them by the new 

 process. Their first attempts indeed met with three principal difficul- 

 ties which, for a long time, were regarded as insurmountable or at 

 least as certainly placing photography at a disadvantage with respect 

 to visual observation. 



The first difficulty was due to the imperfect achromatism of the 

 objectives, until recently always designed for direct vision so as to 

 bring to the same focus the brightest rays of a star without taking into 

 account the rays which have the greatest chemical activity. 



The second difficulty was due to the lack of a suitable control mech- 

 anism which would assure a persistent agreement between the displace- 

 ment of the telescope and the apparent movement of the focal image of 

 the object. 



Finally, the lack of sensitiveness of photographic plates made it 

 necessary to have recourse to relatively long exposures, which increased 

 all the displacement due both to a defective motion of the telescope and 

 to atmospheric undulations. 



The first difficulty was overcome by MM. Paul and Prosper Henry, 

 who demonstrated the possibility of increasing the photographic efll- 



