THE HORSESHOE NEBULA IN SAGITTARIUS. 273 



cions, which are of weight in in-oi)orti(jn as they accumulate ; and, lastly, when 

 practicahle, correcting by comparison of the judgments of ditferent persons at 

 the same time. 



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

 able objects by thus laying a gruundwork 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 nebula, 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. 



"In nebuke of great extent, however correctly estimated may be the stars 

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

 nebula a*e 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 of 

 the larger stars are well settled by tixod instruments, there will be far less room 

 for error in estimations which spread, as from so many centres, over the remain- 

 ing intervals. 



AV 



Fi(i. 3 Mason, 1839. 



" I Vv^ill here speak of a method that 1 hit upon for the exact representation of 



nebulae, which has essentially contributed to the accuracy of the accompanying 



delineations. It was first suggested by the method usually adopted for the 



representation of heights above the sea-level on geographical nu^ps, by drawing 



VOL. viii. IS 



