J 
108 Prof. Gautier on recent Researches relating to Nebule. 
odically, and he thinks it is now possible, though with less cer- 
tainty, to affirm that the same is true of nebule. 
Sir W. Herschel has divided the nebule into three classes 
according to the degree of light. D’Arrest has found a great 
number of cases where nebula, such as had been first classified 
by Herschel, ought now to be displaced one or even two units 
in the classification. The latter cause has changed, in the course 
of a number of years, many of his own estimates of the magni- 
tude of nebule. But, in view of the great diversity of atmo- 
. influences in moist climates, for observations of this kind, 
Arrest agrees with Otto Strnve that it is not possible to be so 
eonfident in regard to conclusions deduced from variations of 
this kind. Meanwhile he states a small number of cases where 
he has been able to show some positive variation. 
_ The first case of this kind is one deduced from observations 
of Struve upon the nebula of Orion which I have mention 
above. The observations upen this nebula recently made by 
d’Arrest, and frequently repeated, with his great telescope, 02 
favorable nights, have confirmed those of Struve, especially those 
relating to the bridge upon the ‘ Sinus Magnus,” which has been 
frequently visible at Copenhagen the past winter, (1861-2,) and 
it has appeared just as it was Salectibeil by Lassell. 
he second case of well marked variation is the almost total 
disappearance of a small and feeble nebula discovered by Hind, 
Oct. 1ith, 1852, in the eonstellation Taurus, recognized by other 
astronomers and easily discernable, at the commencement 0 
1856, with a telescope of 6 feet focal distance. ‘Two years later 
star at 9°4. Its magnitude was no more than the 10th in 1858, 
the 11th in 1861 and only the 18th or 14th magnitude in Feb- 
ruary, 1862. i 
Sir John Herschel thought that he lately found another ex 
ample of the disappearance of a nebula, not seeing inscribed 1p 
the first catalogue of d'Arrest a very feeble nebula described by 
Sir W: : i 
. Herschel near two others in the Hair of Berniee, But 
Chacornac with the aid of the telescope of Foucault proved that 
this feeble nebula was still visible, and d’ Arrest has aiso 0 ed 
it with his great telescope. This astronomer mentions also 4 
small number ef cases where there may have been a variation of 
