306 Neighbourhood of the Lunar Spot, Mare Crisium. 



an E. and W. direction. For instance, the face of a cliff 

 may, from this cause, in one lunation be visible in feeble illu- 

 mination; in another, at the same age of the moon, it may be 

 entirely darkened itself, and even cast a perceptible line of 

 shade. Gruithuisen pointed out the effect thus produced on 

 the dimensions of the shadow of the lunar Apennines, but 

 it seems not to have been duly allowed for by Schroter. 



While the direction both of incident and reflected light 

 is thus continually changing from the combined effect of the 

 two librations, it is easy to see how optical illusions may be 

 of continual occurrence, and how objects whose diversified 

 planes and angles render their aspect peculiarly dependent 

 upon the mode of illumination, may often exhibit themselves 

 in a strangely altered guise ; and the fact that even the 

 maxima and minima of libx*ation are subject to some amount 

 of change, although slight, from the peculiarly variable 

 character of the lunar orbit, introduces still greater difficulty 

 into the attempt to eliminate, from observed appearances, 

 this fruitful source of uncertainty and deception. 



The student will, it is hoped, regard with indulgence this 

 disquisition, which has extended itself far beyond the original 

 intention, and which goes in part over ground traversed upon 

 a former occasion. To some readers it may appear very un- 

 interesting. But, as far as I have observed, the differing 

 character and combined result of the two librations have not 

 been fully and distinctly elucidated in the ordinary treatises 

 on elementary astronomy, though it is of especial importance 

 that the subject should be clearly understood by the seleno- 

 graphies! student : in no other way can he form a just notion, 

 from his own observations, of the probability of actual change 

 in progress on the lunar surface, or of its being subject to 

 at mospheric obscuration ; and in no other way could wo expect 

 to bring to any satisfactory conclusion a comparison of the 

 labours of our predecessors. In the present, as in many another 

 instance, B. and M. challenge the supposed variations of 

 Schroter. On die grounds just assigned, implied rather than 



distinctly explained by them, their position may be defended, 

 that the alleged changes may bo accounted for without the 



supposition of physical alteration. On the other hand, in 



behalf of Sehrot cr's idea, his own argument may be adduced, 



that such merely optical effects must be confined within narrow 



limits, or tin; whole surface would appeal- to bo in a state 

 of fluctuation and uncertainty, and changes would supervene 



in the course of a few hours' observation ; experience showing, 



on the contrary, a great and general uniformity, whioh renders 



the occasional exceptions the more remarkable : to which may 

 be added that all such variations must bo periodical, and would 



