EARL ROSSE’S OBSERVATIONS ON THE NEBULA. 
505 
I 
possible with our present means. The nebula itself, however, is pretty well studded 
with stars, which can be distinctly seen of various sizes, and of a few of these, with re- 
ference to the principal nucleus, measurements were taken by my assistant, Mr. John- 
stone Stoney, in the spring of 1849, during my absence in London; for some time 
before the weather had been continually cloudy. These measurements have been 
again repeated by him this year, 1850, during the months of April and May. Justus 
w^as the case last year, in February and March the sky was almost constantly over- 
cast. He has also taken some measures from the centre of the principal nucleus to 
the apparent boundary of the coils, in different angles of position. The micrometer 
employed was furnished with broad lines formed of a coil of silver wire in the way I 
have described, seen without illumination. Some of the stars in the nebula are so 
bright, I have little doubt they would bear illumination ; if so, their positions with 
respect to some one star might be obtained with great accuracy of course by employ- 
ing spiders’ lines ; this season however it is too late to make the attempt. Several of 
these stars are no doubt within the reach of the great instruments at Pulkova and at 
Cambridge, U. S., and I hope the distinguished astronomers who have charge of them 
will consider the subject worthy of their attention. Their better climate gives them 
many advantages, of which not the least is the opportunity of devoting time to mea- 
surements without any serious interruption to other work. I need perhaps hardly 
add, that measurements taken from the estimated centre of a nucleus, and still more 
from the estimated termination of nebulosity, are but the roughest approximations ; 
they are however the only measurements nebulosity admits of, and if sufficiently 
numerous, I tbink they will bring to light any considerable change of place, or form, 
which may occur. 
The spiral arrangement of 51 Messier was detected in the spring of 1845. In the 
following spring an arrangement, also spiral but of a different character, was detected 
in 99 Messier, Plate XXXV. fig. 2. This object is also easily seen, and probably a 
smaller instrument, under favourable circumstances, would show everything in the 
sketch. Numbers 3239 and 2370 of Herschel’s Southern Catalogue are very pro- 
bably objects of a similar character, and as the same instrument does not seem to 
have revealed any trace of the form of 99 Messier, they are no doubt much more 
conspicuous. It is not therefore unreasonable to hope, that whenever the southern 
hemisphere shall be re-examined with instruments of great power, these two re- 
markable nebulae will yield some interesting result. 
The other spiral nebulie discovered up to the present time are comparatively diffi- 
cult to be seen, and tbe full power of tbe instrument is required, at least in our 
climate, to bring out the details. It should be observed that we are in the habit of 
calling all objects spirals in which we have detected a curvilinear arrangement not 
consisting of regular re-entering curves; it is convenient to class them under a com- 
mon name, though we have not the means of proving that they are similar systems. 
They at present amount to fourteen, four of which have been discovered this spring; 
there are besides other nebulie in which indications of the same character have been 
3 T 
MDCCCL. 
