PHOTOGRAPHY OF THE LUNAR SURFACE. Ill 



between 1828 and 1837 without being able to distinguish the two for- 

 mations from one another except by their coordinates. To-day they 

 differ from one another both in form and in dimensions, and their dis- 

 similarity, unanimously recognized, is so pronounced as to strike even 

 the least attentive observer. l 



The phenomenon described above, in the case of Alpetragius d., is 

 repeated in the case of Linne, a white spot situated in the " Sea of 

 Serenity." 2 Biccioli, Lohrmann, Madler, and Schmidt all agree in rep- 

 resenting in this place a very deep crater, from 6 to 10 kilometers in 

 diameter and quite visible. Schmidt himself, in 1866, pointed out the 

 disappearance of this crater. Since this epoch the spot of Linne only 

 shows ;in extremely small opening, bearing no resemblance to the old 

 descriptions. 



It would be easy to extend this list by citing numerous examples 

 taken principally from Scroter and Gruithuisen. We have only 

 described those cases which are based on independent and concordant 

 testimony. Indeed, in such matters the sources of error are very 

 numerous, and the selenographers of former days do not appear to have 

 sufficiently guarded against them. The sun, moon, and earth are only 

 found in the same relative positions at rare intervals. A change of a 

 few degrees in the position of the circle of illumination, or in the decli- 

 nation of the earth relative to the lunar equator, modifies in a marked 

 degree the aspect of the formations which are adjacent either to the 

 terminator or to the center of illumination. The part due to these 

 influences being known, it remains well established by photographs, 

 which are to-day in our possession, that the old drawings are no longer 

 true. Do the differences correspond to effects really observed 1 ? It 

 seems almost certain that there is no escape from this conclusion in the 

 case of Messier, and very probably in that of Linne. In the latter case, 

 however, there is some room for doubt, for although several drawings 

 indicate changes, there are others which might be cited which contra- 

 dict them, and hence make them open to suspicion. To show how 

 uncertain conclusions thus established may be, it would be easy to find 

 drawings of Mars made on the same day and at the same hour by two 

 different observers in which even the most important markings could 



1 Since this verification is the most precise we possess, we will quote from the 

 writings of Beer and. Madler, whose authority in selenography is so great and so 

 well founded: "To the east there arises a crater similar to the first in all respects 

 in form, height, depth, color of the interior and of the rampart, and even in the 

 position of certain summits on the latter. The entire agreement is so pronounced, 

 that it must have been due to a singular stroke of chance, unless some unknown law 

 of nature has thus manifested itself. We are certain that since 1829 we have seen 

 this region, as we have just described it on more than three hundred occasions; in 

 other words, as often as it could be observed. In a formation so well characterized 

 the least changes of size, shape, or luminosity would have made themselves visible, 

 the observations of Schroter having induced us to keep a sharp watch on this 

 locality." 



- Mare Serenitatis. 



