Page:Popular Science Monthly Volume 8.djvu/292

278 his own telescope had opened with its list of six hundred new nebulæ. And it may be 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 inserting 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 drawing nebulæ, is the following. 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 motion 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 paper 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 micrometric 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 ruled 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 present, 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 obtained, 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 Horseshoe Nebula made in this way by M. Trouvelot, of Cambridge, Massachusetts, the artist to whom we owe the exquisite plates of astronomical engravings published by Harvard College Observatory, under the superintendence of its late director, Prof Winlock.