Page:Popular Science Monthly Volume 8.djvu/287

Rh , which are of weight in proportion as they accumulate; and, lastly, when practicable, correcting by comparison of the judgments of different persons at the same time.

"The assistance which is rendered to the faithful description of these remarkable objects by thus laying a groundwork of stars, may be well illustrated by the familiar expedient of artists, who divide any complicated engraving which they would copy, into a great number of squares, their intended sketch occupying a similar number. The stars, which are apparently interwoven throughout the whole extent of the nebula, furnish a set of thickly-distributed natural points of reference, which, truly transferred to the paper, are as available as the cross-lines of the artist in limiting and fixing the appearance of the future drawing.

"In nebulæ of great extent, however correctly estimated may be the stars immediately around the standard of reference, those in the distant parts of the nebula are liable to suffer from an accumulation of errors of nearly the same kind as that arising in an extended trigonometrical survey. But if the places of the larger stars are well settled by fixed instruments, there will be far less room for error in estimations which spread, as from so many centres, over the remaining intervals.



"I will here speak of a method that I hit upon for the exact representation of nebulæ, which has essentially contributed to the accuracy of the accompanying delineations. It was first suggested by the method usually adopted for the representation of heights above the sea-level on geographical maps, by drawing