^73-] Changes in the Moon's Surface. 493 



it was a depressed circle surrounded by a ring, and distin- 

 guished from the neighbouring objects under all angles by- 

 its dark grey tint. Subsequently, it often appeared, even in 

 a 27-foot reflector (mirror about 20 inches) under favourable 

 circumstances as a bright longish flat mountain, though 

 more frequently in its original grey aspect ; occasionally it 

 would be so indistinct, other objects being well defined, that 

 he could not tell what to make of it. On March 1, 1797, 

 Alhazen being very near the limb (only 27*27 " from it), and 

 therefore in a position to be very indistinct, especially as the 

 terminator had advanced to the other side of the Mare, he 

 saw it with a 13-feet reflector (mirror about 9 inches and 

 power 180) more distinctly than ever, and in quite a new 

 form, as a real very deep and bright crater, with an irregular 

 ring, scarcely united to the south, and open to the north, 

 with a projection on the east side. Also, there was a small 

 shadow as of a crater never seen before during the 

 innumerable observations of ten years. Schroter thought 

 Alhazen, under this aspect, appeared as deep as Proclus." 

 " Mr. Webb," proceeds Mr. Birt, " enclosed a tracing, with 

 this remark : — ' I think you will consider it as affording an 

 interesting comparison with your own observations. He 

 has figured, you will see (as well as described), a little crater 

 where you describe two (?). The circumstance of his ranges 

 uniting so closely to the south may be due to a different 

 Iteration.' Kunowsky, in the Astronomische Jahrbuch for 

 1825, speaks of Alhazen as lost. In the Jahrbuch for 1826, 

 Pastorff, writing January 20, 1823, says his son repeatedly 

 found Alhazen. Pastorff also saw it. In the Jahrbuch for 

 1827, Harding is recorded as having seen Alhazen as 

 Schroter had drawn it. Pastorff saw it in the same year, 

 and in 1829. The difference of aspect, as well as the occa- 

 sional difficulty of finding this interesting spot, is highly 

 curious. Schroter's earliest delineation gives it, as described, 

 a shallow depression, entirely surrounded by a ring. My 

 own observations, which now follow, may perhaps throw 

 some light on these differences and difficulties. Two 

 ranges of mountains, one behind the other, may easily be 

 taken for a crater, and at a certain angle of illumination it 

 may be exceedingly difficult to distinguish the difference. 

 After a while the supposed crater entirely vanishes, libration 

 alters the visual angle, and rotation the illuminating one, 

 the observer being greatly puzzled as to what has become of 

 his well-recognised crater." 



Mr. Birt then describes the observations made by himself 

 on January 3 and 4, 1862. On the former day Schroter's 



vol. in. (n.s.) 3 s 



