274 ^HE POPULAR SCIENCE MONTHLY. 



curves which represent horizontal sections of hill and valley at successive eleva- 

 tions above the level of the sea that is, by lines of equal height and it is the 

 same in its principle. It is obvious that, if lines be imagined in the field of 

 view, winding around through all those portions of a nebula which have exactly 

 equal brightness, these lines, transferred to our chart of stars, will give a faith- 

 ful representation of the nebula and of its minutiae, and of the suddenness as 

 well as of the amount of transition from one degree of shade to another. 



" By far the greatest obstacles to the successful comparison of modern obser- 

 vations on nebulge with those which own, at least, a brief antiquity, exist in the 

 want of precision with which the labors of former observers have been con- 

 ducted, and hence all attempts to trace the slow progress of their changes end 

 in uncertain conjectures and conflicting probabilities. I shall not, therefore, 

 incur the charge of unnecessary minuteness in endeavoring to render, by every 

 means, our knowledge of the present form and state of at least these few nebu- 

 IjB, as far as possible, standard ; and, although laden with the necessar^^ imper- 

 fections of original observations, yet fi'ee from adventitious and unnecessary 

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

 observer, those slight particulars which a chart canuot easily urge upon the 

 notice of any but the original compiler, and further, to indicate the degree of 

 certainty with which difl:erent features of the nebulis were recognized, 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 cer- 

 tain^'' 'nearly certain^'' 'strongly suspected,^ and 'slightly suspected.'' Thus 

 much for observation for rendering the idea of the object as perfect as may be 

 in the mind of the observer. For the most unimpaired communication of this 

 idea or perception, the theory of the process adopted is briefly 1. To form an 

 accurate chart of all stars capable of micrometrical measurement in and around 

 the nebula. 2. From these, as the greater landmarks, to fill in with all the 

 lesser stars, down to the minimum visibile by estimation, which, with care, need 

 not fall far short of ordinary measurement. 3. On this, as a foundation, to lay 

 down the nebula. 



" The first intention was to intrust entirely to careful estimation the copying 

 of the stars which were to form the groundwork of the nebula, since no means 

 of measurement were then at hand. The following is a sketch of the course of 

 ])rocedure adopted in pursuance of this plan : The limits of the nebula were 

 traced as far as long and close examination could discern them, and a rough 

 chart was made of the principal stars within it. This preparation was indispen- 

 sable, because, in the consequent mapping down of all the visible stars in the 

 nebula, it was necessary to use a light out-of-doors, and the object, of course, 

 became invisible. The distance between any two conspicuous stars favorably 

 situated in the nebula was then chosen as a standard of reference ; and, from 

 this as a base, a kind of triangulation was carried out by the eye to all the stars 

 in the neighborhood, and these were successively marked on a sheet of paper at 

 the time ; their magnitudes were also affixed to each according to a fictitious 

 scale, for which a few stars, conveniently situated, furnished standards of i-efer- 

 ence as to size. A lamp was close at hand, whose light could be cut off at 

 pleasure ; and almost direct comparison was thus instituted between the stars 

 in the field of view and those on the paper, and corrections made where any 

 distortions in the latter were observable. As tlie work advanced from night to 

 night, the reference to the lamp was necessarily less and less direct, since a 

 longer exclusion of light was necessary to see the fainter stars. Finally, the 

 nebula itself was drawn upon the map by the guidance of the stars already 



