THE HORSESHOE NEBULA IN SAGITTARIUS. 275 



cojjied ; and although only an occasional and nnfrequent reference could be 

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

 currence, that the memory could as easily as before retain its estimates of dis- 

 tance and direction, until mutual comparison could be made between the map 

 and the heavens." 



It will be seen what a great advance had been made in the concep- 

 ^tion of the application of the topographical method of contour lines 

 to the delineation of degrees of brightness, although this method has 

 practical limitations not spoken of by Mason, and we must consider 

 the careful separation of the various results into classes ranged accord- 

 ing to their degrees of certainty, as scarcely less important. In all 

 former memoirs the chart included all the results reached, and there 

 was no searching division of these in such a way as to give absolute 

 data to the future investigator. 



Throughout the entire memoir (which relates also to other nebulae 

 than the one now in question) the whole endeavor is to reach a per- 

 fect definiteness of conception ; and Mason evidently held the idea 

 that, in the existing state of astronomy, it was eminently "better to 

 do one thing well than many things indifferently." 



Fig. 4. Lamont, 1837. 



Lamont tells us in Annalen der K. Sternxcarte bei Mimchen, Band 

 xvii. (1868), that his early researches on this and other nebula? were 

 prosecuted in the hope that something might be determined as to their 



