I have waited, therefore, before writing this, until from my own observation, or from a drawing carefully executed by Dr. Klein, I might ascertain the exact place of the new crater. I could not, as it turned out, observe the new crater as a black spot myself, since the question was raised; for on the only available occasion I was away from home. But I now have before me Dr. Klein's carefully drawn map. In this I find the new crater placed not nearly, but exactly where Rutherfurd's crater appears. I say "Rutherfurd's crater," for the white spot is manifestly not merely a light tinted region on the darker background of the Sea of Vapours (as the region in which the crater has been found is called): it is a circular crater more than two miles in diameter; and the width of the crescent of shadow surrounding its eastern side shows that in March 1865, when Rutherfurd took that photograph, the crater was not (for its size) a shallow one, but deep.
Now, it is quite true that, to the eye, under high illumination, the floor of the crater does not appear lighter than the surrounding region; at least, not markedly so, for to my eye it appears slightly lighter. But everyone knows that a photograph does not show all objects with the same depth of shading that they present to the naked eye. A somewhat dark green object will appear rather light in a photograph, while a somewhat light orange-yellow object will appear quite dark. We have only to assume that the floor of the supposed new crater has a greenish tinge (which is by no means uncommon) to understand why, although it is lost to ordinary vision when the Sea of Vapours is under full illumination, it yet presents in a photograph a decidedly lighter shade than the surrounding region.
I ought to mention that the writer from whom I have quoted says that all the photographs were examined and the different objects in this region identified within forty-eight hours of the time when Dr. Klein's letter reached England. He mentions also that he has himself personally examined them. Doubtless at that time the exact position of the supposed new crater was not known. By the way, it is strange, considering that the name Louis Rutherfurd is distinctly written in large letters upon the magnificent photograph in question, that a selenographer who has carefully examined that photograph should spell the name Rutherford. He must really not assume, when on re-examining the picture he finds the name spelled Rutherfurd, that there has been any change, volcanic or otherwise, in the photograph.
In conclusion I would point out that another of these laborious crater-counters, in a paper recently written with the express purpose of advocating a closer and longer-continued scrutiny of the moon, makes a statement which is full of significance in connection with the subject of lunar changes. After quoting the opinion of a celebrated astronomer, that one might as well attempt to catalogue the pebbles on the sea-shore as the entire series of lunar craters down to the minutest visible with the most powerful telescope, he states that while on the one hand, out of thirty-two thousand eight hundred and fifty-six craters given in Schmidt's chart, not more than two thousand objects have been entered in the Registry he has provided for the purpose (though he has been many years collecting materials for it from all sides); on the other hand, "on comparing a few of these published objects with Schmidt's map, it has been found that some are not in it,"—a fact to which he calls attention, "not for the purpose of depreciating the greatest selenographical work that has yet appeared, but for the real advancement of selenography." Truly, the fact is as significant as it is discouraging,—unless we are presently to be told that the craters which are not common to both series are to be regarded as new formations.
Richard A. Proctor, in Belgravia.
FOOTNOTES:
[63] To some this may appear to be a mere truism. In reality it is far from being so. If two globes of equal mass were each of the same exact temperature throughout, they might yet have very unequal total quantities of heat. If one were of water, for instance, and the other of iron or any other metal, the former would have far the larger supply of heat; for more heat is required to raise a given weight of water one degree in temperature, than to raise an equal weight of iron one degree; and water in cooling one degree, or any number of degrees, would give out more heat than an equal weight of iron cooling to the same extent.