i6o THE POPULAR SCIENCE MONTHLY. 



Notliing more was ever seen of tlie light, so far as any record 

 informs us, until 1865, when Grover, an English observer, caught 

 sight of it again, under circumstances similar to those of its 

 first apparition, and watched it for half an hour, when it once 

 more disappeared. It should be said that, in the case of Dr. Ger- 

 ling's observation, referred to by Prof. Holden, a " small, round, 

 isolated, conical mountain " was found in the place where the 

 light had been, on the evening following its appearance. It is 

 altogether probable that the gray or black spot perceived by 

 Schroeter was the shadow of a similar mountain, for it is well 

 known that some of the lunar mountains and hills are hardly 

 visible at all except when lateral illumination indicates their po- 

 sition and form by means of the shadows. 



Herschel thought he had seen three active volcanoes. If 

 Prof. Holden's discovery accounts for one of these, it is possible 

 that the observations I have just described may give a clew to the 

 others. The phenomenon seen by Schroeter and Grover was located 

 fifty or sixty miles north of the point where Prof. Holden beheld 

 the extraordinary blaze of light last July, and at a point where 

 the mountains, drawing around a culminating peak, confront with 

 tremendous buttresses the broad level of the Mare Imbrium. 



The objection has been made by Messrs. Elger and Williams, 

 two competent English observers, that Herschel's volcanoes can 

 not be identical with the glittering peaks seen by either Holden 

 or Gerling, because the latter were observed close to the line of 

 sunrise, where the morning rays touched them, while the phe- 

 nomena that attracted Herschel's attention were situated far 

 within that part of the disk where the only light came from the 

 earth. But Prof. Holden does not say that the illumination he 

 witnessed was identical in pZace with those recorded by Herschel, 

 but simply that it was identical in Icind. Besides, it must be re- 

 membered that, if these luminous appearances are due to peculiar 

 angles of reflection, a similar effect must be produced whether the 

 reflecting surfaces are presented to the sunlight or only to the 

 earth-shine. The difference would be simply in the degree of 

 brightness of the phenomena. 



But while the discovery with the Lick telescope may ac- 

 count for Herschel's mistake, it does not clear up the mystery of 

 the cause of these extraordinary lights. In every case quoted 

 above, the illumination was evidently very much greater than 

 that of Aristarchus, the most brilliant of the shining mountains. 

 Proctor estimated that the reflective power of Aristarchus must 

 be equal to that of new-fallen snow. But the mountain-crest ob- 

 served by Prof. Holden blazed with a dazzling brilliancy that it 

 would be difficult to account for except upon the theory that 

 nearly all of the sunlight falling upon it was reflected to the ob- 



