SCIENTIFIC SUMMARY. 
517 
been hitherto very satisfactory — the mottled and luminous streaks in the 
neighbourhood of the spots being shown by those of M. De la Rue — yet it 
would appear that the time of exposure must be carefully attended to, or 
the difference between the photographs and the eye-pictures becomes very 
apparent. In those taken at Ely by Professor Selwyn, the small exposure 
adds to the clearness of the fainter features of the sun, whilst with those 
taken at Kew, a long exposure blots out all the minute details. This 
latter is, however, purposely done, as it is only to obtain sharp definition 
of the margins and spots that observations are carried on at Kew. Mr. 
De la Rue attributes the better definition of the details of the facuhe 
to the fact of the Kew instrument being in really better chemical 
focus. In all the very minute features, however, the eye must be de- 
pended on. 
Comet of December 28 th } 1860. — Dr. Micliez, of the Observatory of 
Padua, has calculated an elliptic orbit of this comet, which, it will be 
remembered, was considered as identical with those of 1490 and 1810. It 
would seem, however, that its identity with the last comet is doubtful, as 
a period of about 108 years satisfies the observations of the present year. 
This period represents also very imperfectly the comet of 1490, although 
the other elements are not far wrong. As this, however, is the principal 
one, the identity must be considered as yet doubtful. 
Relation of Periodic Times . — Mr. Finlayson, of Dover, points out the 
following singular proportion :-—The period of rotation of the earth on 
its axis is in the same proportion to the periodic time of moon round the 
earth as that of the period of rotation of the sun on its axis is to the 
periodic time of Mars round the sun. 
Nebula of Eta Argils and Southern Double Stars. — Mr. Powell observing 
at Madras, confirms his own and Mr. Abbot’s previous suspicions of the 
changes in the nebula surrounding Eta Argus. He states that since 1860, 
the whole nebula has faded away very considerably, and has also altered 
its form ; the star now being left quite isolated, and out of the nebulous 
matter altogether. It is to be remembered that the star is one of the most 
remarkable variable ones, and the phenomenon of the star and nebula 
varying in brightness at the same time resembles that of the disappear- 
ance of the small star and telescopic nebula of Taurus a few years since. 
Mr. Powell has also made some good observations on the remarkable 
star, Alpha Centauri, which is now at an interesting part of its orbit, as 
the distance is now decreasing. Gamma Coronee Australis he finds has 
described 20 degrees during the last four years. 
Early Observations of Uranus. — Two early observations of Uranus have 
been discovered by Mr. Hugh Breen in Dr. Bradley’s unpublished Green- 
wich Observations. The dates are 1748, Oct. 21, and 1750, Sept. 13. At 
the latter time it was near Alpha Capricorni. On both occasions it is 
noted as being of the sixth magnitude. 
Chinese Record of Eclipses. — Out of the thirty- six solar eclipses which 
Mr. Williams has extracted from the Chinese records, the Astronomer 
Royal has found thirty-two to agree with those in the Art de Verifier les 
Dates. Two appear impossible ; but Mr. Williams has found that one of 
those is an erroneous repetition. Two are not real eclipses, being probably 
