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 are. His 
paper was perhaps the most important one of the session. In it 
was presented a new method of 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 Astronomical 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 Linné. Ile 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, there is a com- 
paratively open region which has been often studied. Schroter, 
Lohrmann, Gruithuisen, Beer and Midler, Schmidt, Klein, Neison, 
have all made this region the object of special study, 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 region 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. 
‘“What 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 1876 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 1876 
