[35] HERSCHEL accepted, as did all his cotemporaries, the Newtonian or corpuscular theory of light.
[36] Thus the position of small stars critically situated in the centre, or on the edges of the nebulæ was always noted. Many of the descriptions are given in the published papers, but the publication of the diagrams would be an immense help to this branch of astronomy. D'ARREST in his reduction of HERSCHEL'S nebula observations (1856) writes: "Gewiss wäre es vom höchsten Interesse für die Entwickelung, welche hoffentlich auch dieser Zweig der beobachtenden Astronomie zukünftig erhalten wird, wenn die HERSCHEL'Schen Beobachtungen in der Ausführlichkeit in welcher sie, verschiedenen Andeutungen zufolge, _handschriftlich_ vorhanden sind, veröffentlicht würden. Es schliesst sich dieser Wunsch in Betreff der Nebelflecken lebhaft an den an, welcher, schon vor einem Jahrzehnt nach Veröffentlichung der 400 noch unedirten _star-gauges_ von gewichtigerer Seite her geäussert wurde." In this all must agree who have a knowledge of the direction in which we must look for advances in the difficult and important questions of the distance, the motions, and the changes of the nebulæ. Almost the only aid to be looked for from the older observations must come from such diagrams, and we may safely say that the publication of this priceless material, just as it stands, would carry our exact data back from 1833 to 1786, or no less than forty-seven years.
[37] Long after HERSCHEL had abandoned this idea, it continued current among astronomers. The successes of Lord ROSSE'S telescope perpetuated to the middle of the nineteenth century an erroneous view which HERSCHEL had given up in 1791.
[38] These have never been re-observed. They should be sought for with a powerful refractor, taking special precautions against the illumination of the field of view from neighboring bright stars. HERSCHEL'S reflectors were specially open to illusions produced in this way. His observations probably will remain untested until some large telescope is used in the way he adopted, _i. e._, in sweeping.
I.--LIST OF THE PUBLISHED WRITINGS OF WILLIAM HERSCHEL ON ASTRONOMICAL SUBJECTS.
_N.B.--In general, translations and abstracts of those which appeared in periodicals are not noticed here. I have made exceptions in the more important cases._