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234 had been watching, and still greater changes since Huyghens, a century and a half earlier, gave a picture of it in his Systema Saturnium. "The various appearances of this nebula," Herschel writes, "are so instructive that I shall apply them to the subject of the partial opacity of the nebulous matter. . . . For when I formerly saw three fictitious nebulous stars, it will not be contended that there were three small shining nebulosities, just in the three lines, in which I saw them, of which two are now gone, and only one remaining. As well might we ascribe the light surrounding a star, which is seen through a mist, to a quality of shining belonging to that particular part of the mist, which by chance happened to be situated where the star is seen. If then the former nebulosity of the two stars which have ceased to be nebulous can only be ascribed to an effect of the transit or penetration through nebulous matter which deflected and scattered it, we have now a direct proof that this matter can exist in a state of opacity, and may possibly be diffused in many parts of the heavens without our being able to perceive it."

It would be unjust to Herschel to pass over the condemnation of his views, pronounced by Sir David Brewster in his Life of Sir Isaac Newton, Without mentioning the name of William Herschel, or of La Place, who advocated the same views. Sir David writes as one who felt sure that Newton, for mathematical reasons alone, would have taken a side against this Nebular Hypothesis. In the last of the famous four letters written by Sir Isaac to Dr. Bentley, the great classical scholar and the author of Phalaris, he enters