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234 ophthalmometer, or an instrument for measuring the eye, which has proved of much service. His next researches were on the field of spectro-analysis, and he demonstrated that the results obtained by painters in mixing colors do not correspond with those of mingling the pure spectral colors of solar light or of other lights decomposed with a prism. He showed, for example, that, when yellow and blue rays of light are combined, the color produced is white, and not green. In connection with this subject, he entered into an admirable analysis of the extent and limits of human observation, and totally refuted at the same time Sir David Brewster's alleged decomposition of solar light.

In 1855 Helmholtz was called to the University of Bonn as Professor of Anatomy and Physiology, and director of the Anatomical and Physiological Institute. Three years later he removed to the University of Heidelberg, to fill a chair of Physiology, which he continued to occupy for more than twelve years. It would be tedious to enumerate every thing that his patient labor and the unerring logic of his mind demolished, while there, in the snug but shaky edifices of conservative science. He scattered the results of his researches with a liberal hand all over Germany. Every scientific periodical was honored by him with a contribution on its own specialty. There is, however, one class of his articles that appeared during this period which must not be overlooked. He had rendered valuable services toward an exact understanding of the mechanism of the eye, and had observed the physical laws of vision. He now turned his attention to a subject of equal importance. He investigated the mechanism of the ear, and searched for the laws of sound. His discoveries are laid down in his "Doctrine of the Sensations of Sound, as a Physiological Basis for the Theory of Music." It contains a complete analysis of the conditions of harmony, and reduces its aesthetic principles to a few fundamental physical laws.

Helmholtz's new doctrines of sight and sound have been universally received, and his fame will last as long as they continue to be among the main pillars of physical science. His name is also inseparably connected with the doctrine of the conservation of force, which subsequent investigation at the hands of others has shown to be the key-note of every law of Nature. Its bearing on the doctrine of evolution is, however, so strong and favorable that, like that doctrine, it will need much time before it receives an unreserved acceptance. Helmholtz tested, a few years ago, the progress which the doctrine of evolution has made, by asking the congress of natural philosophers, assembled at Speier, to declare, openly, who of them were in favor of, and who against, Darwinism. The roll was called, and there was not one against it. Helmholtz removed, in 1871, to Berlin, and holds there a professorship in the University.