276 The Lunar Eratosthenes and Copernicus. 



does not, however, refer to this) was not unacquainted with 

 such appearances, having noticed them several times in 

 craters (he especially mentions Gopernicus, Theophilus, Zach, 

 and Sacrobosco), where an ill-defined edge of brownish grey 

 rendered the length of the shadow difficult of measurement, 

 while other perfectly similar craters in the same neighbour- 

 hood were free from any such peculiarity. For an explanation 

 he sees no need of having recourse to penumbra or atmosphere ; 

 it might be sufficiently accounted for by a multitude of colossal 

 blocks on the crest of the ring, whose narrow lines of shadow 

 cast upon the opposite wall, with their intervening streaks of 

 light, being separately undistinguishable by us, would produce 

 the confused general impression of a diluted border. In this 

 case, he refers to some terrestrial correspondences, such as the 

 shadow of a row of close-set iron spikes on the top of a door or 

 wall, seen from a suitable position and distance ; or the shadow 

 of a fringe of ice-ruins and rock-pinnacles falling on a snowy 

 slope, which he once remarked in great beauty from the Wen- 

 gern Alp ; the edge of a mountain shadow, elsewhere sharp, 

 becoming very indistinct when projected on the very obliquely 

 slanting snows of the Silverhorn and Gruggi glacier, the employ- 

 ment of a common eye-glass showed the cause to lie in the 

 confused impression of many long, narrow, separate streaks of 

 shade. (We may observe, by the way, that the eye for terres- 

 trial scenery, evident in this great observer, qualifies him in a 

 high degree for the analogical interpretation of the varied 

 aspects of the Moon.) 



But though this is a plausible elucidation, it may not be 

 the true key to the mystery. To the objection that it would 

 be difficult to account in this way for so broad a zone of duski- 

 ness as Schr. has represented, falling, too, upon a slope inclined 

 in the wrong direction, it might be answered that his drawing- 

 was too rough to be trusted in minute details, and that he 

 seems to have satisfied himself too easily upon the subject. 

 But it is evident that more has yet to be explained, and that 

 the point deserves study. Though there is no improbability in 

 the idea that the summit of a ring should be crested with a 

 row of natural battlements or pinnacles ; yet these, if close 

 enough to produce a confused half-tone in the part of the 

 shadow cast by the ridge where it lies facing the sun, would in 

 every other position overlap one another so much in perspec- 

 tive as to intercept too much light, and produce a full and 

 defined shade. Here, therefore, a very powerful instrument 

 would so far decide the question that an equal intensity of half- 

 tone along the whole border of the shadow would negative 

 Schmidt's solution. And so would any want of periodical 

 recurrence in the phenomenon — a point which seems to have 



