Hersehel's Catalogue of Neb nice. 417 



that, in comparison with the full five thousand enumerated, the 

 proportion of suspected nebulae is not large, and that a great 

 number of such instances may be accounted for, without the 

 supposition of erroneous entries, by their extreme faintness 

 and the uncertainty of our own atmospheric conditions, in 

 addition to the dissimilarity of eyes and instruments. Even 

 in the experience of a single observer, and that one of the 

 highest class, Sir J. Herschel, we are told that " very great 

 differences will occur in the descriptions of one and the same 

 nebula, taken on different nights, and under different atmo- 

 spheric circumstances, as well as in different states of the 

 mirror and the eye ; nor will it at all startle one accustomed to 

 observations of nebulae to see such an object described at one 

 time as ' faint, small, round/ and at another as ' bright, 

 pretty large, pretty much extended, resolvable/ " It would, 

 indeed, be but natural to dispose of all the recorded variations 

 in this way, but for the tolerably well ascertained fact, or, at 

 least, the strong presumption, that some three or four nebulae 

 have actually exhibited variations of light ; and this mar- 

 vellous and perplexing disclosure gives wide entrance to 

 any amount of suspicion with regard to the rest ; especially 

 when we bear in mind that, from the analogy of variable stars, 

 the rate of change may be extremely diversified, and in man} r 

 instances exceedingly slow ; and that the period during which 

 they have received minute and repeated attention — much less 

 than a century — is comparatively a short one. The existence 

 and the aspect of a little patch of hazy light, so feeble as 

 perhaps to escape any but the more powerful instruments, 

 were of course in the first place facts, and nothing more, 

 and attracted little notice in proportion ; it was only when 

 unexpected results began to disclose themselves, that they 

 acquired significance and importance. We have now reached 

 another stage in the inquiry ; and since two wholly inde- 

 pendent modes of investigation, by direct telescopic exami- 

 nation, and by analyzation of light, seem to converge to- 

 wards conclusions of the most remarkable nature, every 

 suspicious appearance becomes worthy of notice and record. 

 Changes of three different kinds have previously been sus- 

 pected among these objects — of place, of form, and of 

 brightness. Those of place are very questionable, usually dis- 

 appearing when more correct determinations of position are 

 obtained : we find, however, three cases in the Catalogue in 

 which such an alteration may be suspected, either in nebulae or 

 in the stars adjacent to them.* Those of form embrace four 



* Keferences to position are not given in these cases, because the objects are, 

 generally speaking, too faint for general observation. The possessors of suffi- 

 ciently powerful instruments should be provided with the Catalogue. 



