207 



that future changes in their appearance, should any occur, may be 

 detected. The process employed was — 1st. To prepare an accurate 

 chart of all the stars in and about the nebula, capable of micrometri- 

 cal measurement. 2dly. To fill in with the smaller stars down to the 

 minimum visibile, by estimation. 3dly. To lay down the nebula on 

 this chai't with such care and precision, that the errors of its delinea- 

 tion may not far exceed those of original vision. The author, Mr. 

 Mason, states at length the expedients used to effect the latter pur- 

 pose, viz. — the drawing of lines of equal brightness, as a guide to 

 the engraver ; the examination of each portion of the nebula by seve- 

 ral persons; and, lastly, the repeated comparison of the drawings 

 with the original, on successive evenings, till no further improvement 

 seemed to be practicable. 



The telescope, used by Messrs. Smith and Mason, was of their own 

 construction. It was 14 feet in length, and had 12 inches clear aper- 

 ture, being a Herschelian, mounted somewhat rudely on the plan of 

 Mr. Ramage. The difficulties experienced by Messrs. Smith and 

 Mason, as amateur artists in casting and polishing specula at New 

 Haven, are stated in detail. The telescope was capable of sepa- 

 rating f Orionis, p? Bootis, y Virginis in 1838, AOphiuchi, and others 

 of a distance of less than 1". For such purposes, however, the use 

 of diaphragms was necessary, owing to an imperfection of the cast- 

 ing, and the full light of the telescope could not be employed. This 

 circumstance directed their attention to the subject of this paper. 



A cursory examination of the principal nebulas described, and, in 

 some instances, figured by the Herschels, pointed out discrepancies 

 between their descriptions and present appearances, which must be 

 attributed either to a change in the nebulas themselves, or to the want 

 of sufficient minuteness of examination on the part of the Herschels, 

 whose object was rather the formation of a complete catalogue of the 

 nebulas in the heavens, than the full and perfect description of any of 

 the individuals. Thus, the paper contains a drawing of the "nebula 

 trifida," h. 1991: the triple star does not occupy the same position in 

 the cleft as given in the figure in Sir J. F. W. Herschel's paper, Phil. 

 Trans. 1833, but rather adheres to the left of the three divisions; 

 and what is more remarkable, the small star about 30' north of this 

 triple star was surrounded with a nebula not much inferior in size 

 and brilliancy to the " nebula trifida." A drawing is also fur- 

 nished of the nebula, h. 2008, (the shape of which resembles the 



