Oautier on Nebulae. 421 



M. Gautier then proceeds to speak of the labours of M. 

 d' Arrest, first with a 4^ inch telescope, at Leipsic, and since 

 then with an 11 inch achromatic at Copenhagen, which has 

 enabled him to discover more than 100 new nebulas. With 

 regard to variable nebulae he thinks much caution needed. 

 He confirms Struve's observations on the nebula of Orion, and 

 finds Mr. Hind's little nebula in Taurus to be also variable.* 

 The nebula which Sir J. Herschel thought had disappeared, 

 has been seen by M. Chacornac with the Foucault telescope, 

 and by M. d' Arrest with his great reflector. 



Sir J. Herschel, in his great paper on nebulas (published in 

 Phil. Trans., 1833), remarked that the number of nebulae 

 physically united, is probably larger in comparison with the 

 total number of nebulae, than that of double stars when com- 

 pared with the entire number of known stars ; and taking 5' 

 as the maximum distance of double nebulas, M. d* Arrest has 

 already arranged fifty under that category, and he estimates 

 that there are two or three hundred of this sort out of about 

 three thousand nebulae visible in our hemisphere. 



M. d' Arrest mentions a triple nebula, but only in one 

 instance has he been able to observe noticeable changes indicat- 

 ing a common revolution. The interesting object has a Right 

 Ascension of 109° 12', and N. Declination 20° 45'. It is repre- 

 sented by M. Lassell, in the engraving accompanying his 

 memoir, in vol. xxiii. of the Astronomical Society. Its two 

 components are distinct, being only separated by 28', but they 

 are difficult to see when the micrometer threads are illumi- 

 nated. A very small star is seen between them, exactly where 

 M. Lassell observed it ten years ago. 



Finally, M. d'Arrest reports a small number of instances in 

 which a slight change of distance and position has been noticed 

 after the lapse of a certain time, between particular nebulae and 

 small adjacent stars. 



We may remark that, if it can be shown that two or more 

 nebulae, each probably a system of thousands of suns arranged 

 in a particular order, and at great distances from each other, 

 really revolve about one another, or about a common centre, 

 not only will some of the grandest views of creation be opened, 

 but fresh speculations will arise concerning the nature of the 

 force by which such mighty movements are compelled. 



* See Intellectual Observes, toI. ii., page 310. 



M. Auwers has published some observations made by him at Gottingen and 

 Konigsberg, to show that the two last variable nebulas of M. d' Arrest have not 

 really altered in luminosity. With regard to the nebula of Mr. Hind, which is 

 the only one whose periodic variation has been proved, the same astronomer states 

 that he saw it perfectly in March, 1858, but that it was feebler than in 1856. M. 

 Chacornac could not find it in 1858, and erroneously thought it had disappeared. 



