38 Pi^oceedings of the American Philosophical Eociety. 



The object of Messrs. Smith and Mason was to furnish a minute de- 

 scription of some of the principal nebulae in the heavens, in order 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 micrometrical measurement. 

 2dly. To fill in with the smaller stars down to the minimum visibile, by 

 estimation. 3dly. To lay down the nebula on this chart with such care 

 and precision, that the errors of its delineation may not far exceed those 

 of original vision. The author, Mr. Mason, states at length the expedi- 

 ents used to effect the latter purpose, viz. the drawing of lines of equal 

 brightness, as a guide to the engraver ; the examination of each portion 

 of the nebula by several persons ; and lastly, the repeated comparison of 

 the drawings with the original on successive evenings, till no further im- 

 provement seemed to be practicable. 



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

 construction. It was fourteen feet in length, and had twelve inches clear 

 aperture, 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 separating t Orionis, 

 l-i^ Bootis, y Virginis in 1838, ^ Ophiuchi, 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 casting, 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 nebulae 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 nebulae 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 nebulae 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 trifda" 

 A drawing is also furnished of the nebula, h. 2008, (the shape of which 

 resembles the capital Greek Q,) with a critical examination of Herschel's 

 figure of the same. 



The most remarkable discovery of Messrs. Smith and Mason, was that 

 of the junction of the two nebulaj, h. 2092 and 2093. These great 

 nebulae, or "milky ways" are described on several occasions by the elder 



