PHOTOGRAPHY OF THE LUNAR SURFACE. 109 



to collect during a number of years; moreover, the quantity of data is 

 of less value than their certainty. Delineations and descriptions of 

 difficultly visible objects are in a certain measure necessarily a product 

 of interpretation and memory. Their agreement is even very fre- 

 quently not a guarantee of the fidelity of the observer, often influenced 

 by preconceived opinions. The conditions in photography are entirely 

 different. Instead of being temporary, as is the case with the retinal 

 image, the chemical impression is permanent and can be rendered 

 indestructible. A certain control can be assured by making, as is 

 nearly always done, a number of exposures which no personal prejudice 

 can alter. They escape all physiological and mental influences which 

 might make the hand or the judgment of the artist deviate from the 

 truth. 



Another cause which aggravates the difficulties of direct vision is 

 due to the presence of a complex tableau which is being continually 

 transformed. The sensitive plate furnishes a faithful image corre- 

 sponding to a given epoch. 



Lack of time prevents the portrayer not from perceiving the details, 

 but from reproducing all of them, and the subject even experiences 

 important modifications before the work of delineating is completed. 

 Delicate markings might be altered by the inevitable impurities of the 

 sensitive films, of the developing baths, or of the wash water, but such 

 alterations are easily recognized, at least as long as they do not affect 

 details bordering on the limit of visibility. In any case, however, 

 errors which might be introduced can easily be eliminated by making 

 a number of exposures at short intervals apart. 



It can therefore be truly affirmed that photography, having become 

 an indispensable auxiliary for stellar astronomy, can also render impor- 

 tant services in the physical study of the surfaces of our planets. 

 The moon, much nearer to us than any other celestial body, would, 

 reasoning in this manner, be a most fertile field for discoveries. Dur- 

 ing the last few years the most powerful telescopes of the Lick Observ- 

 atory, in California, and of the Paris Observatory have been utilized 

 in its study. The collections of negatives of the two observatories, 

 taken together, inclose the elements of a complete atlas or our satellite 

 (at present in course of publication). Not only do the> reveal a multi- 

 tude of details outside of those which are enumerated and represented 

 on lunar maps, but they also furnish a certain basis for the recognition 

 and analysis of the variations which may be wrought by time on the 

 surface of our satellite. 



V. — SOME OF THE PRINCIPAL CHANGES WHICH HAVE SUPPOSEDLY 

 BEEN OBSERVED ON OUR SATELLITE. 



The importance of a verification of such changes is extreme, and 

 selenographers appear to have early recognized this. Fearing natu- 

 rally to get lost in the infinite multitude of details, they have devoted 



