EARL ROSSE’S OBSERVATIONS ON THE NEBULAE. 
503 
The same considerations also forbid any more minute description of the telescope 
and its mounting. 
From what has been said, it is evident that the 6-feet specula being occasionally 
in a state of strain, were not uniform in their action. There was however another 
cause of unequal action. The 6 -feet specula, after they have been polished, cannot 
be tested till they have been removed from the laboratory to the telescope, there to 
await a good night, the great focal length making it impossible to test them while 
on the engine. Now it has often happened that a speculum which has subsequently 
proved to be incapable of very fine definition, has remained in the telescope during 
a succession of moderately good nights, when a great deal of work was done, await- 
ing a night when the air was in a state to warrant a decisive opinion. Such a spe- 
culum might do good work, but it would not resolve difficult nebulae, neither would 
it bring out faint points of light, even when wide apart. There is still another cause 
of the unequal action of our specula far more serious, the varying state of the atmo- 
sphere. When the air is unsteady, minute stars are no longer points, the diffused 
image is much fainter, and single stars, easily seen when the air is steady, are no 
longer visible. When many minute stars are crowded together the whole become 
blended, and instead of a resolved nebula we have merely a diffused, perhaps bright 
nebulosity. The transparency of the air varies also quite as much ; and the aspect of 
the nebulae changes from night to night, just as the appearance of a distant building 
alters as the details of the architecture are more or less obscured by the intervening 
mist. With these facts, the Society will not be surprised should it be in our power at 
a future time to communicate some additional particulars, even as to the nebulae 
which have been the most frequently observed. 
The sketches which accompany this paper are on a very small scale, but they are 
suffieient to convey a pretty accurate idea of the peculiarities of structure which have 
gradually become known to us : in many of the nebulae they are very remarkable, 
and seem even to indicate the presence of dynamical laws we may perhaps fancy to 
be almost within our grasp. To have made full-sized copies of the original sketches 
would have been useless, as many micrometrical measures are still wanting, and there 
are many matters of detail to be worked in before they will be entitled to rank as 
astronomical records, to be referred to as evidence of change, should there hereafter 
be any reason to suspect it. 
Much however as the discovery of these strange forms may be calculated to excite 
our curiosity, and to awaken an intense desire to learn something of the laws which 
give order to these wonderful systems, as yet, I think, we have no fair ground even 
for plausible conjecture ; and as observations have accumulated the subject has be- 
come, to my mind at least, more mysterious and more inapproachable. There has 
therefore been little temptation to indulge in speculation, and consequently there 
can have been but little danger of bias in seeking for the facts. When certain phe- 
nomena can only be seen with great dififtculty, the eye may imperceptibly be in some 
