OBSERVATIONS OF LTTNAR OBJECTS. 301 



sideration, that wheiiever close attention is given to the moon's surface some- 

 thing of the kind is sure to crop up ; nor is it confined to eye observations 

 alone ; photography tells the same tale. In the letterpress to the fourth area 

 of the Lunar Map I have given instances of differences between photograms 

 of various dates ; and in drawing up my monogram of Hipparchus, I com- 

 pared every object in every available photogram. It is much to be regretted 

 that a means of detecting differences, if not changes on the moon's surface, 

 should be so little utilized, for I have not met with any published results of 

 the comparison of lunar photograms except my own, as specified above ; and 

 we know that numerous negatives must be in existence. We can hardly 

 conceive it possible that illusion can enter as an element here ; apparent dif- 

 ferences may result from flaws either in the originals or in printing, but these 

 are capable of being eliminated : and, again, on photograms we have whatever 

 differences may present themselves under the eye ; whereas in those observed 

 with the telescope we have the records only to depend upon, and these 

 records ^viU be more or less convincing according to the impression made by 

 the phenomena on the mind at the time. As illustrative of this I quote Mr. 

 Pratt's remarks in connexion with the phenomena of November 20, 1871 : — 

 " Whether or no Lunar Meteorology ever becomes an accepted fact, I shall 

 always retain a strong belief that this observation was one of the earliest and 

 most complete records I know of, from the greatest intensity of the mist, or 

 whatever it was that obscured the region, until its entire dissipation by the 

 rays of the rising sun." With this Mr. Pratt contrasts an observation of the 

 Mare Prigoris on December 27, 1871, as follows : — " Definition of objects on 

 the Mare Frigoris fully as good as on any part of the border of Plato, in 

 marked distinction to the observation of November 20, 1871." 



One of the results of my late discussion of observations of the floor of Plato 

 is, that certain peculiar phenomena, consisting of variations' of the brightness 

 of the N.W. floor and in the forms of the streaks thereabout, have been 

 noticed during the greater part of two years by two or more independent 

 observers. On the 22nd of December, 1871, Mr. Pratt noticed " a marked 

 haziness over the north-west part of the floor of Plato, an instance of very 

 limited mistiness." Still, comparison with other portions of the floor rendered 

 it to Mr. Pratt's mind a no less certain instance than the former one of the 

 Mare Frigoris ; for to one who has so constantly worked at the floor, even 

 limited phenomena would be as apparent as those of wider range to the 

 general observer. 



Now what are we to say to illusion ? Here are independent observers 

 during a period of many months testifying to the existence of the same phe- 

 nomena ; and not only so (for their testimony would have been weak had we 

 merely taken a disjointed remark here or there, or had one observer only, as 

 in the case of Schroter, recorded these seeings), but we have had the ob- 

 servations carefully examined and arranged under certain heads, the evidence 

 has been sifted, and we think that an impartial verdict would negative illu- 

 sion, and declare for some active element producing the phenomena observed. 

 What that element is becomes t. most interesting question. So far as we 

 have been able to make out, the most active agency that has modified the 

 moon's surface is volcanic. Have the appearances to which allusion has been 

 made any connexion with a continuance of this agency ? 



