noted for the first time, so far as I know: this recession is a real phenomenon, and is thus spoken of by Sir John Herschel: "The nebula which is very bright in the parts surrounding the trapezium, seems (whether by the effect of contrast with the dazzling light of these stars, or from a real deficiency in nebulous matter) to have retreated from immediate contact with them, so that they appear in some degree insulated and with a darkness about them. This would agree with the idea of a subsidence of the nebula into the stars by gravitation; but it is probably only a deception."
Fig. 4.
Nebula Orionis. (Herschel, 1824.)
Sir John likewise expresses an opinion as to the resolvability of the nebula, and describes its appearance in a very graphic way. He says: "I know not how to describe it better than by comparing it to a curdling liquid, or to a surface strewed over with flocks of wool, or to the breaking up of a mackerel sky when the clouds of which it consists begin to assume a cirrous appearance. ..." It presents, however, "no appearance of being composed of small stars, and its aspect is altogether different from that of resolvable nebulæ. In the latter we fancy by glimpses that we see stars, or that, could we strain our sight a little more, we should see them. But the nebula suggests no idea of stars, but rather of something quite distinct from them."
In the beginning of 1834 Sir John Herschel went to the Cape of Good Hope for the purpose of completing a survey of the whole heavens, which had been commenced in England: he took with him his 20-foot reflector (aperture 18 1/4 inches), and devoted himself during
