When HERSCHEL first began to observe the nebulæ in 1774, there were very few of these objects known. The nebulæ of _Orion_ and _Andromeda_ had been known in Europe only a little over a hundred years.
In 1784 MESSIER published a list of sixty-eight such objects which he had found in his searches for comets, and twenty-eight nebulæ had been found by LACAILLE in his observations at the Cape of Good Hope. In the mere discovery of these objects HERSCHEL quickly surpassed all others. In 1786 he published a catalogue of one thousand new nebulæ, in 1789 a catalogue of a second thousand, and in 1802 one of five hundred. In all he discovered and described two thousand five hundred and eight new nebulæ and clusters. This branch of astronomy may almost be said to be proper to the HERSCHELS, father and son. Sir JOHN HERSCHEL re-observed all his father's nebulæ in the northern hemisphere, and added many new ones, and in his astronomical expedition to the Cape of Good Hope he recorded almost an equal number in the southern sky.
Of the six thousand two hundred nebulæ now known the HERSCHELS discovered at least eight-tenths. The mere discovery of twenty-five hundred nebulæ would have been a brilliant addition to our knowledge of celestial statistics.
HERSCHEL did more than merely point out the existence and position of these new bodies. Each observation was accompanied by a careful and minute description of the object viewed, and with sketches and diagrams which gave the position of the small stars in it and near it.[36]
As the nebulæ and clusters were discovered they were placed in classes, each class covering those nebulæ which resembled each other in their general features. Even at the telescope HERSCHEL'S object was not discovery merely, but to know the inner constitution of the heavens. His classes were arranged with this end, and they are to-day adopted. They were:
CLASS I. "Bright nebulæ (288 in all). II. "Faint nebulæ (909 in all). III. "Very faint nebulæ (984 in all). IV. "Planetary nebulæ, stars with burs, with milky chevelure, with short rays, remarkable shapes, etc. (79 in all). V. "Very large nebulæ (52 in all). VI. "Very compressed and rich clusters of stars (42 in all). VII. "Pretty much compressed clusters (67 in all). VIII. "Coarsely scattered clusters of stars" (88 in all).
