42 TRANSACTIONS OF THE [dEC. 7, 



Professor Michelson described his recent measurements of Jupiter's 

 satellites at the Lick Observatory, and thought that we may hope 

 to measure the angular diameters of some of the brighter stars, if 

 they be as great as the hundredth part of a second of arc. His 

 paper was perhaps the most important one of the session. In it 

 was presented a new method oi measuring the angular diameters 

 of luminous discs by means of the interference phenomena produced 

 by them. The experiments made at the Lick Observatory have 

 been described in the Publications of the Adronoviical Society of 

 the Pacific. The 12-inch telescope was used, but a telescope is by 

 no means indispensable for these observations, the chief requisite 

 being a very favorable condition of atmosphere. It is to be hoped 

 that these very promising researches will be continued. 



III. Changes on the Lunar Surface. 

 BY J. K. reEs. 



(Abstract.) 



Professor Rees first presented briefly the various reported dis- 

 coveries of changes on the lunar surface, making especial mention 

 of the case of Linne. lie then stated the case of Hyginus N. " Let 

 us recapitulate the present state of this most important question. 

 In the central regions of the moon, where libration has no sensible 

 effect in altering the appearance of the lunar surface, thei'e is a com- 

 paratively open region which has been often studied. Schroter, 

 Lohrmann, Gruithuisen, Beer and Miidler, Schmidt, Klein, Neison, 

 have all made this region the object of special stud}', so that it has 

 been repeatedly examined with most powerful instruments. A 

 great number of very small crater-hills and ridges have been seen, 

 and all drawn and mapped. A smaller number of larger craters 

 and hills have also been seen, and every one which existed has been 

 mapped. So often has this reg-ion been studied, that several years 

 ago it was pointed out as being one of the very few regions in which 

 if a change occurred it would be possible to prove it. 



" 'V^hat do the present observations show ? This, that in the very 

 midst of this region there now exists a big black, rounded spot, per- 

 haps a crater, perhaps merely a depression, possibly only a surface 

 marking, but in any case so large and conspicuous as to be almost 

 the most distinct object in the entire region. We know that the 

 minuter features shown on the maps and drawings made anterior 

 to 1816 could only have been seen and drawn within forty-eight 

 hours of sunrise and sunset. We know, moreover, that most of 

 these drawings were made shortly after sunrise Yet the present 

 observations would indicate that now, whenever these smaller de- 

 tails are visible, there, in their very midst, stands this great black 

 object. For more than thirty-six hours after sunrise this black, 

 crater-like object appears to be now always visible. Before 187(' 



