379 
SCIENTIFIC SUMMARY* 
QUARTERLY RETROSPECT. 
ASTRONOMY. 
Figure of the Sun . — A photographic apparatus has been projected by- 
Mr. De ia Rue, by which it is intended to obtain pictures of the sun, 
in order to examine whether its disc be really circular, and more especially 
to determine whether it is flattened at the poles. The Astronomer Royal, 
however, considers this question to be settled, and is of opinion that the 
proposed instrument would not add any information to our knowledge 
on the subject. The enormous mass of measures of the solar diameter 
recorded at Greenwich, and other observations, would, he holds, be more a 
test of the trustworthiness of the instrument rather than the reverse. In 
fact, from twenty-five years’ observations at Greenwich of the horizontal 
and vertical diameters of the sun (where 2,487 measurements were made 
of the former and 2,694 of the latter), it has been found that the hori- 
zontal exceeds the vertical diameter by only one-tenth of a second of arc — 
a quantity almost imperceptible. 
Missing Nehida. — The evidence in favour of changes of form, and 
variability in the light of nebulse, has hitherto been regarded with con- 
siderable suspicion by astronomers. That a single star should fluctuate in 
brightness presents sufficient difficulties in the way of explanation — that 
a whole swarm of stars could he extinguished at one and the same time, 
and not only it, but a distinct star in its neighbourhood — -presents 
obstacles which are almost insurmountable in the actual state of our 
knowledge. The few nebulae which Sir W. and Sir J. Herschel found to be 
missing whilst engaged in their surveys of the heavens were very naturally 
attributed to wrong entries, or to the original body having been a comet, 
both of which mistakes might have easily occurred. Even Sir W. 
Herchel’s assertion that he had actually perceived changes in the nebula of 
Orion between 1780 and 1811 (which was, as Arago observes, avoir pris 
la nature sur le fait), and when he made use of the same telescope on 
both occasions, was looked upon with doubt by his illustrious son, 
although it has been verified of late years by M. Otto Struve. At 
present there can he no further doubt concerning the actual disappearance 
of a nebula. The object in question was situated at R.A. 4h. 13m. 
N.P.D. 70° 50', and was discovered by Mr. Hind in 1851. It was again 
independently found by M. Chacornac in 1854, and observed by M. 
