504 
EARL ROSSE’S OBSERVATIONS ON THE NEBULAE. 
degree influenced by the mind ; therefore a preconceived theory may mislead, and 
speculations are not without danger. On the other hand, speculations may render 
important service by directing attention to phenomena which otherwise would escape 
observation, just as we are sometimes enabled to recognize a faint object with a small 
instrument, having had our attention previously directed to it by an instrument of 
greater power. The conjectures therefore of men of science are always to be invited 
as aids during the active prosecution of research. 
It will be at once remarked, that the spiral arrangement so strongly developed in 
Plate XXXV. H. 1622,51 Messier, fig. 1 , is traceable, more or less distinctly, in several 
of the sketches. More frequently indeed there is a nearer approach to a kind of irre- 
gular interrupted annular disposition of the luminous material than to the regularity so 
striking in 5 1 Messier ; but it can scarcely be doubted that these nebulse are systems of 
a very similar nature, seen more or less perfectly, and variously placed to the line of 
sight. In general the details which characterize objects of this class are extremely faint, 
scarcely perhaps to be seen with certainty on a moderately good night with less than 
the full aperture of 6 feet: in 51 Messier, however, and perhaps a few more, it is not 
so. A 6-feet aperture so strikingly brings out the characteristic features of 51 Mes- 
sier, that I think considerably less power would suffice, on a very fine night, to bring 
out the principal convolutions. This nebula has been seen by a great many visitors, 
and its general resemblance to the sketch at once recognized even by unpractised 
eyes. Messier describes this object as a double nebula without stars ; Sir ^Villiam 
Herschel as a bright round nebula, surrounded by a halo or glory at a distance 
from it, and accompanied by a companion ; and Sir John Herschel observed 
the partial subdivision of the s.f. limb of the ring into two branches. Taking 
Sir J. Herschel’s figure, and placing it as it would be if seen with a Newtonian 
telescope, we shall at once recognise the bright convolutions of the spiral, which 
were seen by him as a divided ring. We thus observe, that with each successive 
increase of optical power, the structure has become more complicated and more 
unlike anything which we could picture to ourselves as the result of any form 
of dynamical law, of which we find a counterpart in our system. The connection 
of the companion with the greater nebula, of which there is not the least doubt, and 
in the way represented in the sketch, adds, as it appears to me, if possible, to the dif- 
ficulty of forming any conceivable hypothesis. That such a system should exist, 
without internal movement, seems to be in tlie highest degree improbable: we may 
possibly aid our conceptions by coupling with the idea of motion that of a resisting 
medium ; but we cannot regard such a system in any way as a case of mere statical 
equilibrium. Measurements therefore are of the highest interest, but unfortunately 
they are attended with great difficulties. Measurements of the points of maximum 
briglitness in the rnotling of the different convolutions must necessarily be very loose ; 
for although on the finest nights we see them breaking up into stars, the exceedingly 
minute stars cannot be seen steadily, and to identify one in each case would be im- 
