170 . OBSERVATIONS ON NEBULA 



nebula were traced as far as long and close examination could discern thera, 

 and a rough chart was made of the principal stars within it. This preparation 

 was indispensable, 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, 

 favourably 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 neighbourhood, 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 reference as to size. A lamp was close at hand, whose light could 

 be cut off at pleasure; an almost direct comparison was thus instituted between 

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

 any distortions in the latter were observable. As the 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 

 copied; and although only an occasional and unfrequent reference could be 

 made to a lamp, the stars within it had become so familiar by their constant 

 recurrence, that the memory could, as easily as before, retain its estimations of 

 distance and direction, until mutual comparison could be made between the 

 map and the heavens. 



The assistance which is rendered to the faithful description of these remark- 

 able objects by thus laying a groundwork of stars, may be well illustrated by 

 the familiar expedient of artists, who divide any complicated engraving which 

 they would copy, into a great number of squares, their intended sketch occu- 

 pying a similar number. The stars, which are apparently interwoven through- 

 out the whole extent of the nebulse, furnish a set of thickly distributed natural 

 points of reference, which, truly transferred to the paper, are as available as 

 the cross-lines of the artist in limiting and fixing the appearance of the future 

 drawing. 



8. In nebulse of great extent, however correctly estimated may be the stars 

 immediately around the standard of reference, those in the distant parts of the 

 nebula are liable to suffer from an accumulation of errors of nearly the same 

 kind as that arising in an extended trigonometrical survey. But if the places 



