from that excellent master, Müller afterward spoke in the most grateful terms, and declared that it was through the influence and example of Rudolphi alone that his own scientific pursuits were afterwards turned so fully in the direction of comparative anatomy.
At the expiration of his two years of labor, and immensely enriched in all the fields of natural science, Müller again returned to Bonn, and in 1824 was enrolled as academic lecturer in comparative anatomy and physiology. Two years later, when but twenty-five years old, he was made professor extraordinary in the same branch of science.
The epochs in his activity in investigation which immediately followed upon his return to Bonn have well been called by DuBois Reymond the subjective physiologico-philosophical period. The literary landmarks of this period in Müller's career are two works: First, "On the comparative physiology of the sense of sight in men and animals, with researches on the motions of the eyes and on the sight of man"; second, "Concerning the phantasmal phenomena of vision: a physiological research dealing with the physiological evidence of Aristotle concerning dreams, the philosophies and the arts."
In the former of these two works we find recorded that excellent discovery that the sight of insects (which possess facet-eyes) must be conceived of as a mosaic interpretation of objects; that is, the pictures which the insects themselves see are placed together as in the form of a mosaic. In the second work regarding the "Phantasmal Phenomena of Vision," Müller took up a study, the idea of which reached far back into his earliest youth, when he was accustomed to give free play to his fancy in imagining strange shapes and figures on the plaster-scarred walls of the old buildings. These fanciful appearances, which thus early became so familiar in the imaginings of his boyhood, he submitted in maturer years to searching philosophic scrutiny; and the work in which they are described and discussed is a charming yet masterly application of experiment in anatomy, physiology, physics and psychology. Through the medium of these scientific principles Müller explained the seeing of devils and spirits; the friar, who, after long hours of supplication, sees the desired consecration in the form of a shining cloud; the superstitious, to whom the tempter appears as an evil spirit: these phenomena were for Müller only the results of the passion-aroused conditions in the material substances of their sight.
Of all Müller's labors at this time, greatest importance must be attached to his work in elucidation of the laws of the specific energy of the sense organs. With ingenious experiment he worked out the general law that, in whatever manner a sense organ may be stimulated, it always answers to our consciousness by the method peculiar to it. It was from these and other related investigations that Müller deduced many of his philosophical principles: For instance, that we can not