304 EECENT RESEAECHES RELATIVE TO THE NEBULiE. 



the bridge over the great strait, which, last winter, was Bometimes distinctly 

 visible at Copenhagen, presenting the appearance assigned it by M. Lassell. 



The second case of well-established variability is the almost total disappear- 

 ance of a small and faint nebula? discovered by M. Hind, October 11, 18-32, in 

 the constellation Taurus, recognized by other astronomers, and in the begin- 

 ning of 1S<36 still readily perceptible with a telescope of six feet focal length. 

 Two years later it was no longer to be seen, except with great difficulty, in the 

 heliometer of the observatory of KiJnigsberg. It was invisible October 3, 1861, 

 with the gi-eat telescope of Copenhagen. M. Chacornac, with the new telescope 

 of M. Foucault, and M. Lassell, at Malta, with his reflecting telescope of four 

 feet diameter, sought for it in vain in 1862, though it was still to be seen with 

 the great achromatic telescope of Poulkova. 



A curious circumstance, connected with the great diminution of the bright- 

 ness of this nebula, is that this diminution has coincided with that of a small 

 star ^^'hich presented itself almost in contact with the nebula. M. Argelander 

 estimated the magnitude of this star, in 1852, at 9.4. It was of not more than 

 the tenth magnitude in 1858, of the eleventh in 1861, and of the thirteenth or 

 fourteenth in 1862. 



Sir John Herschel believed that he had recently detected another instance 

 of the disa])pearance of a nebula from not having found inscribed in the first 

 catalogue of M. d'Arrest a very faint nebula, noticed by Sir W. Il-erschel, near 

 two others in the constellation of Berenice's hair. But M. Chacornac has ascer- 

 tained, with the new telescope of M. Foucault, that this feeble nebula is still 

 plainly visible, and M. d'An-est has also observed it with his great telescope. 

 The latter astronomer further cites a small number of cases where there may 

 have been variability of brightness, or even disappearance of nebulai ; but these 

 instances are not as well established as that of the nebula of M. Hind. 



DOUBLE NEBULAE. 



Sir John Herschel has remarked in his important memoir on the nebulae, 

 published in the PMlosopJiical Transactiovs for 1833, p. 502, that the number 

 of nebulai physically connected with one another is probably more considerable, 

 relatively to the total number of the nebulae, than is that of double stars among 

 the fixed stars.* Admitting a mutual distance of five minutes of a degree to 

 be the greatest for the double nebulre, M. d'Arrest even now enumerates about 

 fifty comprehended within this limit, and is of opinion that there may be two 

 or three hundred of them among the whole number of some 3,00'0 nebulae dis- 

 cernible in our heavens, t So considerable a proportion of double nebulaj justi- 

 fies the presumption that there is a real connexion in these groups, and their 

 aspect confirms this idea, particularly in the case where unusual forms present 

 themselves at once in two equal exemplifications. Sir W. Herschel seems not 

 to have had an idea of this physical connexion betAveen the nebulre, but Sir 

 John distinctly speaks of it on more than one occasion. It can scarcely be 

 doubted that at some future period astronomers will be called on to calculate the 

 orbits of the double nebulae. 



M. d'Arrest mentions some particular cases of this sort of nebula?, of which 

 one is triple. There is as yet but one recognized, where, on comparing the 

 distances and respective positions of the two nebulae of the same group observed 

 m 1785, 1827, and 1862, considerable changes have been noticed, which seem 

 to indicate a movement of revolution of the one around the other. This double 



A short analysis of this adinirable paper of Sir J. Herschel, accompanied with a plate, is 

 given in tho issues of the Dibitothcqiic Univcrsdk for June and July, 1834. 



t M. d'Aircst has quite recently published, in No. 13G9 of the A. N., a catalogue, for the 

 commencement of 18tjl, of tho positions and aspect of fifty double uebulse which he has 

 already recognized, and of which a dozen are now ones. 



