278 THE POPULAR SCIENCE MONTHLY. 



his own telescope had opened with its list of six Imndred nem nebulae. 

 And it may he remarked in passing that it is just this intelligent 

 devotion to a definite aim and object which, in this case as in all, has 

 led to brilliant results. We give Lassell's figure above, remarking 

 that it was constructed, as indeed all the preceding ones had been, by 

 first measuring the relative position of the brighter stars, then insert- 

 ing by careful eye estimates the fainter ones, and finally by drawing 

 among these stars, guided by their configurations, the details of the 

 nebula itself. 



Another, and a very rapid method of di-awing nebula?, is the fol- 

 lowing. It yields to the first in the accuracy of the positions of the 

 stars, but it is probably even superior to it in facilities for the correct 

 representation of the nebula and stars considered as one mass. A 

 piece of glass is ruled carefully into squares (see Figs. 6 and 7) and 

 this is placed in the focus of the telescope so as to be plainly visible ; 

 the telescope is then directed upon the nebula, and a clock-work mo- 

 tion is applied to the telescope so that it follows the apparent motion 

 of the nebula from east to west accurately. Some one of the brighter 

 stars is chosen, and it is kept by means of the clock-work accurately 

 in the corner of one of the squares. A piece of i)aper ruled into 

 squares similar to those of the glass reticle is provided, and on it the 

 observer dots down the various stars in and about the nebula. This 

 may take two, three, or four nights according to circumstances, but in 

 all cases it requires much less time than the mici'omctric measurements 

 of the brighter stars and the troublesome allineatious required to fix 

 the positions of the smaller stars, and it has the great advantage that 

 the work can be done in a perfectly dark field of view, whereas the 

 micrometric measures demand the use of illuminated wires at least. 

 After the stars are inserted, the principal lines are put in, not only by 

 the star-groups, but also by the squares themselves. For my own use 

 I have had constructed two reticles : one luled in squares like those 

 seen in Figs. 6 and 7, and another in which the heavy-lined large 

 squares (each containing nine small squares, see Fig. 6) are still pres- 

 ent, but are subdivided into small squares by lines parallel to their 

 own diagonals. After making all the use possible of the first reticle, 

 the second is put in, and an entirely new set of reference-lines is ob- 

 tained, making an angle of 45 with the old set. This, of course, 

 could be equally obtained by revolving the first reticle through an 

 angle of 45, but it is not quite so convenient. 



After the stars and the principal lines of the nebula are inserted a 

 new and higher power eye-piece is used, and the drawing is concluded 

 by means of this. Fig. 6 is an example of a drawing of the Horse- 

 shoe Nebula made in this way by M. Trouvelot, of Cambridge, Massa- 

 chusetts, the artist to whom we owe the exquisite plates of astro- 

 nomical engravings published by Harvard College Observatory, under 

 the superintendence of its late director. Prof Winlock. 



