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Rh and for that reason all experimental philosophy is instituted".

In his paper on the "Construction of the Heavens," in 1785, he wrote, "We ought to avoid two opposite extremes. If we indulge a fanciful imagination and build worlds of our own, we must not wonder at our going wide from the path of truth and nature. . . . On the other hand, if we add observation to observation without attempting to draw not only certain conclusions but also conjectural views from them, we offend against the very end for which only observations ought to be made." This principle he maintained throughout his life. We can trace its operation through all the wonderful series of papers which he communicated to the Royal Society. In regard to the Milky Way, to island universes, to the nature of nebulæ, to double stars, to the Sun and planets, we see him collecting facts, framing hypotheses to account for these facts, and testing them by further facts. He had no fear of propounding theories, nor had he hesitation in withdrawing them. Theories were to him means to an end—the discovery of truth.

The career of Herschel marked an epoch in astronomy. His powerful genius directed the course of the science in the nineteenth century; and modern astronomy still bears the impress of his massive mind. When he began his long career as an observer, astronomy had become more or less a branch of applied mathematics. Such was not to be wondered at; the immediate task of the eighteenth century was the proof of the universal validity of the Newtonian law—a task carried forward to a triumphant conclusion by Lagrange and Laplace. But one result of the concentration of energy on the mathematical side of the science was that the study of the physical condition of the Sun, Moon, and planets was largely neglected; while the stellar branch of astronomy was virtually non-existent.

As a result of his development of the powers of the