200 A. Gautier on recent Researches on Nebulae. 
tive measurements, whether of changes of brightness, or of form, 
or of variations of position, will therefore be most highly inter- 
esting, but they present great difficulties.’’. Mr. Johnston Stoney, 
m 
* 
osse associated with himself in these observations, 
sitions of different stars situated in the nebula, No. 51 of Mes- 
sier, as referred to its central nucleus. The nebula No. 99 of 
Messier, (No. 1173 of Sir John Herschel’s catalogue,) situated 
about 125 10™ of right ascension and 15° of hern declina- 
tion, has also given opportunity for some similar measurements. 
is is the second nebula in which Lord Rosse has shown a very 
distinct spiral structure. He has described also, in his memoir 
of 1850, twelve fainter nebulz of the same class, and he sur 
mised that some others are of the same kind. He described and 
figured in this memoir five new annular nebule, in addition to 
the two already contained in the catalogue of Sir John Herschel; 
some more stars called nebulous, and other nebule of an 
elongated lenticular form, three of which are represented in the 
plates attached to this memoir. 
“ Here,” he says, “in winter celestial objects are usually most distinct, 
and the sky is the darkest before eleven o’clock at night; the sky bow 
ever soon becomes luminous and the details of the nebula which are 
distinct disappear. In spring and autumn the change of light is not 80 
prompt nor so decided, but the nights are shorter. Guided by the admir 
able catalogue of Sir John Herschel (containing the positions and a sum 
mary description of 2306 nebule) we have examined nearly all the rent 
a 
brilliant nebule known, with the exception of a few in the vicinity of 
have not 
itted n made, and the most remarkable objects have been # 
r to a detailed examination, on favorable nights, sometimes ag 
aid of & micrometer. In our eminently variable climate, when 
