WITH A FOURTEEN FEET REFLECTOR. 197 



and this was found to differ considerably from the original map. The number 

 of even revolutions of the micrometer screw may possibly have been read off 

 wrong; and, if this is the case, some eight or ten stars in its neighbourhood 

 will be affected by its error. But these are out of the direct course of the 

 nebula. In general, the relative places are well determined by these right as- 

 censions and declinations. 



Description of the Plates. 



41. By far the greatest obstacles to the successful comparison of modern ob* 

 servations on nebulae with those which own, at least, a brief antiquity, exist 

 in the want of precision with which the labours of former observers have been 

 conducted; and hence all attempts to trace the slow progress of their changes 

 end in uncertain conjectures and conflicting probabilities. I shall not, there- 

 fore, incur the charge of unnecessary minuteness in endeavouring to render, 

 by every means, our knowledge of the present form and state of at least these 

 few nebulse, as far as possible, standard, and, although laden with the neces- 

 sary imperfections of original observations, yet free from adventitious and un- 

 necessary vagueness in the communication of them. In order to supply, to 

 any future observer, those slight particulars which a chart cannot easily urge 

 upon the notice of any but the original compiler, and, farther, to indicate the 

 degree of certainty with which different features of the nebulae v/ere recognised, 

 it is thought proper to bring under this head the enumeration of various facts 

 not expressed in the journal of observations. These are divided into '' things 

 certain" ^^ nearly certain" strongly suspected" and ^^ slightly suspected." 



42. The plates are such as to distort, as little as possible, the relative posi- 

 tion of the stars, since the development of the spherical surface is by projection 

 on a plane tangent to the centre of each nebula. 



The nebulae are in the position in which they appear in a Herschellian re- 

 flector, when on the meridian; '^ south" being uppermost in the present cases, 

 and the preceding, the right hand portions. The letters 5, p, n,f, on the plates, 

 serve as ready references where these terms are used in description. 



In plate V., where one of the nebulae is represented by lines of equal bright- 

 ness, (Article 12,) the half lines serve to indicate suspected gradations of shade, 

 while the even numbers mark those that are more certain. Thus, the lines 1 

 VII. — 2 z 



