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in of the larger organisms which are present in vinegar, cheese and decaying vegetables, and it is not impossible that he may have seen the animal and vegetable cells. The first description of bacteria with illustrations showing their forms was given by Loewenhoeck, a linen dealer in Amsterdam in 1675. The fineness of the linen being determined by the number of threads in a given area, it is necessary to examine it with a magnifying lens, and he succeeded in perfecting a simple lens with which objects smaller than had been seen up to that time became visible. It must be added that he was probably endowed with very unusual acuteness of vision. He found in a drop of water, in the fluid in the intestines of frogs and birds, and in his evacuations, objects of great minuteness which differed from each other in form and size and in the peculiar motion which some of them possessed. In the year 1683 he presented to the Royal Society of London a paper describing a certain minute organism which he found in the tartar of his teeth. After these observations of Loewenhoeck became known to the world they quickly found application in disease, although the author had expressed himself very cautiously in this regard. The strongest exponent of the view of a living contagion was Plenciz, 1762, a physician of Vienna, basing his belief not only on the demonstration of minute organisms by Loewenhoeck which he was able to verify, but on certain shrewdly conceived theoretical considerations. He was the first to recognize the specificity of the epidemic diseases, and argued from this that each disease must have a specific cause. "Just as a certain plant comes from the seed of the same plant and not from any plant at will, so each contagious disease must be propagated from a similar disease and cannot be the result of any other disease." Further he says, "It is necessary to assume that during the prevalence of an epidemic the contagious material undergoes an enormous increase, and this is compatible only with the assumption that it is a living substance." But as is so often the case, speculation ran far ahead of the observations on which it is based. There was a long gap between the observations of Loewenhoeck and the theories of Plenciz, justified as these have been by present knowledge. In the spirit of speculation which was dominant in Europe and particularly in Germany in the latter half of the eighteenth and the first half of the nineteent
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