15 



by varying the illumination either one could be increased at 

 the expense of the other. If the diaphragm is small the 

 illuminating rays approach parallelism and the structure 

 picture is most distinct. If on the other hand you use a 

 large diaphragm and fiood the picture with convergent rays, 

 you make the structure less distinct but the colour becomes 

 much more obvious. Now in examining a section of tissue 

 containing perhaps a few stained Bacteria, the numerous 

 lines and shadows of a complex structure entirely obscure 

 the minute organisms, but when the cone of rays is gradually 

 widened the structure becomes more and more obscured, and at 

 last the coloured organisms become visible. He tells us he 

 did not find the means of accomplishing this satisfactorily 

 until he came upon a condenser arranged by Abbe and 

 constructed by Zeiss of Jena, which gave a pencil of 120° of 

 aperture. He relates that when making experiments on 

 Septicaemia in mice, although he was morally certain that 

 Bacteria were present, he was unable, even after much 

 diligence to find them, until he employed the condenser of 

 Abbe to which he had added a regulated series of diaphragms. 



Beyond matters of detail I do not know that up to the 

 present time any material improvement has been effected in 

 the methods of research in this department. No doubt, as 

 time goes on, further advances will be made, and so the 

 solution of the problem be more nearly approached. My 

 special object in entering into these details was to show that, 

 however difficult it may be to prove the presence of Bacteria, 

 how more so it is to prove their absence, a matter of not less 

 importance in a discussion of this kind. 



Before we go any farther, the question suggests itself 

 naturally. Do Bacteria exist in Normal living Tissues ? This, 

 like every other point on this question, has been fought over 

 very vigorously. The weight of evidence is certainly in 

 favour of the negative answer. Koch holds this opinion, and 

 he is supported by such able observers as Burdon-Sanderson, 

 Pasteur, Klebs, and others. I cannot stop to enter into 

 particulars beyond saying that the battle was fought over the 



