54 Light Spots in the Lunar Night. 



more light, as the object was sufficiently large, lie changed his 

 power for one of 95, but could only now and then perceive an 

 extremely feeble and quite uncertain vestige of it, and after an 

 observation of more than a quarter of an hour, he found it had 

 entirely vanished. Nothing in our own atmosphere could have 

 produced this luminosity, as during the course of nearly half an 

 hour, it had kept its position unmoved, and after its disappear- 

 ance Manilius and Menelaus and the other features of the dark 

 side were as visible as before : it must therefore, he concluded, 

 have been an actual development of light either upon the surface 

 or in the atmosphere of the moon. On comparing its situation 

 with a sketch of the vicinity taken some time before, he found 

 that it concurred with the region of the lunar Alps, which he 

 resolved to study with renewed energy. Here, Oct. 8, he per- 

 ceived, just to the E. of the great peak of Mont Blanc 

 (Int. Obs., ix. 63), and enclosed by the lower ridges of that 

 mountain mass, a round, black, defined spot of 6", like a 

 crater without a ring, all lying in shade, which he never 

 recollected to have seen before : and the measurement of whose 

 position agreed, as nearly as could be expected, with his former 

 estimate. The two following evenings it appeared as a very 

 flat depression, of a darker grey than the Mare. Oct. 11, the 

 grey was only that of the Mare, with a little central darkness, 

 which, 13, had disappeared. 14 and 15, including the full 

 moon, the same. Nov. 8, it was seen again as a round black 

 crater-like hole, though the illumination and libration were 

 nearly the same as on Oct. 9, when it was .only dark grey. 20, 

 though near the line of lunar sunset, and fully exposed to the 

 solar rays, it appeared grey and flat. Dec. 11, the site appeared 

 indistinct in the 4-foot reflector. He details these and many 

 other observations at much length to strengthen his position 

 that something more than mere difference of illumination was 

 concerned in these variations. In this he may probably have 

 been mistaken ; a long smooth slope slightly inclined to the 

 horizon would change its aspect so entirely frcm very trifling 

 variations in the angle of light incident in one direction, and 

 would be so little affected by corresponding variations of illu- 

 mination from the opposite side, that, in$ connection with 

 changes of libration, this cause alone might suffice for all his* 

 recorded phenomena ; yet still they are instructive enough in 

 their own way to deserve this passing notice : and had any 

 regularly-formed crater since been visible upon the spot, we 

 should have acknowledged an evident analogy with the pro- 

 cesses now supposed to be in operation in the crater Linne. 

 Subsequently (1789, Apr. 5), he was much struck by the 

 discovery of a minute bright crater not more than 10" or 12" 

 E. of the supposed dark opening, just where the furthest spur 



