Prof. Gautier on recent Researches relating to Nebula. 109 
brightness and even a disappearance of nebuls, but these cases 
are not as well authenticated as that of Hind. 
Double Nebule.—Sir John Herschel has remarked, in his 
great memoir upon nebula published in the Philosophical Trans- 
achons for 1833, p. 802, that the number of nebule physically 
connected with others is much more considerable, in proportion 
to the total number of nebule, than is the number of double 
stars among the fixed stars." Assumigg a mutual distance of 5 
minutes of a degree as the greatest distance of double nebula, 
d’Arrest has already computed about 50 comprised within this 
limit, and he has estimated that there may be two or three hun- 
had any idea of this physical connection between nebule, but Sir 
John has spoken of it clearly and frequently. There can be little 
doubt that it will be possible, in the distant future, to calculate 
€ orbits of double nebula. 
. d’Arrest mentions some particular cases of nebulz of this 
Sort, one of which is triple. He recognized it only when, on 
ed. 
A Very small star is found between them, exactly at the same 
Place where Lassell found it ten years before. M. d’Arrest will 
Cite hereafter some other analogous cases of change in the rela- 
aye Positions of double nebule, when his work upon this subject, 
determined for some of the double stars. 
Finally d’Arrest describes a very small number of cases where 
6 hee 
«+ brief analysis of t aluable researches of Sir John Herschel, accompanied 
i. Pinte, on per a Payee Bibliothique Universelle for eget July, 1834, 
Arrest i i “A. 7 
Nachrichten” a f 50 double nebule, for 
_citen,” a catalogue of the positions and appearances o : 
ne beginning of 1861, which he has already recognized and of which a dozen are 
