INTRODUCTION. 23 side, he arranged a series of empty test-tubes in the bottom and a pipette in the top, so that when desired the tubes, one by one, could be filled through it. The chamber was first submitted to an optical test to deter- "mine the purity of its atmosphere, and was allowed to stand undisturbed and unused until a powerful ray of light passed through the side windows failed to reflect rays from suspended particles of dust when viewed from the front. When the dust had settled so as to allow the optical test of its purity, the tubes were filled with urine, beef-broth, and a variety of animal and vegetable broths, boiled by submergence in a pan of hot brine; the tubes were then allowed to remain undisturbed for days, weeks, or months. In nearly every case life failed to develop after the purity of the atmosphere was established. In 1873, Obermeier observed that actively motile, flex- ible spiral organisms were present in large numbers in the blood of patients in the febrile stages of relapsing fever. Thus evidence slowly accumulated to establish the theory for which Henle had labored as early as 1821, that for many diseases at least there was a distinct and specific contagium vivum, and the "GERM THEORY" was pro- pounded. Is it not strange that the very idea which was to be the outcome of all this investigation and discussion—an idea which would form a new era in scientific medicine and become a fundamental principle of pathology—was one which had been conceived and taught by a philosopher who lived nearly two thousand years ago ? Among the numerous works of Varrol is one entitled Rerum Rusti- carum libri tres, from which the following is quoted: "Animadvertendum etiam, si qua erunt loca palustria— quod crescunt animalia quaedam minuta, quae non pos- sunt oculi consequi et per aera intus in corpus per os ac n ares perveniunt atque efficiunt difficilis in orb us " (I., xii. 2).—" It is also to be noticed, if there be any marshy 1 Univ. Med. Mag., vol. iii., No. 3, Dec., 1890, p. 152.