PROCEEDINGS OF THE SOCIETY. 281 



diarrhoea," and erroneously regarded as an infectious disease. It was 

 not an infectious disease, and it could not be spread by contagion. It 

 had been ascertained that no breast-fed infant, even amid the most 

 insanitary surroundings, ever contracted the disease. At the Infants 

 Hospital no attempt at isolation was made in the case of an infant 

 suffering from the disease, nor had any infant contracted the disease 

 there, the simple reason for this being that all the babies were fed on 

 raw milk. The organisms producing the disease could not live in raw 

 milk — nor in any acid medium. They were the ordinary organisms of 

 putrefaction, capable of living on any protein matter, and by their 

 metabolic processes speedily produced powerful poisons — the alkaloids. 

 The history of the disease in infancy would be found to be that the 

 baby had been fed on cooked milk, so that the lactic organisms had been 

 destroyed. The typical raw milk organism — the Streptococcus lacticus — 

 was extremely delicate. Growing in pure culture in its most favourable 

 medium, it will die in the course of about a fortnight. The Bacilli/* 

 acidi lactici behaves in the same manner. The B. bulgaricus would live 

 for a little longer — about a month. 



Dr. Vincent then showed photographs of the various organisms 

 stained, unstained, and taken during life. The organisms included the 

 B. subtilis, the B. mesentericus, " No. 7," the B. mesenteric us vulgatus, 

 and the B. proteus vulgaris. Photographs were also shown of the 

 Streptococcus lacticus, the Bacillus acidi lactici, and the B. bulgaricus. 



Dr. Vincent then proceeded with the main subject of his lecture. 



Dr. E. J. Spitta said he would like to congratulate the lecturer on 

 his beautiful photomicrographs, let alone his extremely interesting lec- 

 ture. The essence of success in taking this type of photograph was in 

 correctly suiting your screen to the dye employed as a stain for the 

 bacteria. If too contrasting a screen were employed, then too hard 

 results were obtained, whilst with too little contrast in your filter flat 

 and dirty negatives resulted. He did not know if the lecturer happened 

 to employ the screens made by Dr. Mees — they were sold by Wratten 

 and Wainwright, of Croydon — for with these any class of monochromatic 

 light could be obtained from one end of the spectrum to the other, and 

 any depth of contrast also. If he did not happen to know of these 

 filters he felt sure he would be pleased with their use. 



The only point at which he found himself at variance with Dr. Vincent 

 was in the advantage gained by the employment of the extremely high 

 magnifications he both showed and recommended. Directly magnifica- 

 tion, when using an objective of N.A. 1*40, exceeded 1000 diameters 

 there was necessarily a gradual falling off of defining power, owing to the 

 presence of diffraction phenomena, the circle of confusion slowly becoming 

 greater than r ^ in., and the more the magnification was increased the 

 greater the fuzziness obtained. The reason of this was not far to seek, 

 but as he had dwelt upon that subject at some length elsewhere he would 

 not say more about it. If, however, the primary magnification did not 

 render the image sufficiently large for seeing with comfort — that is to 

 say, without the use of a magnifying glass — then it was very profitable 

 to enlarge the negative, for by so doing the details, although not in- 

 creased in number or refinement, would be rendered more easily seen 



