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reagents are also necessary. It is easy enough in the case 

 of a more or less transparent vegetable or animal infusion 

 to discover Bacteria when they exist in large numbers or are 

 3-ggregated into Zooglaea masses, but it is very difficult when 

 they are isolated and embedded in the meshes of a complex 

 tissue. We are not surprised then to find that many 

 mistakes have been made. Bacteria have been said to be 

 discovered where none existed, and their absence has equally 

 been declared when they were really present. 



It is to Professor Koch that we owe very much of the 

 recent improvements in the methods of investigation, and I 

 ought to say here something about them. 



Von Recklinghausen employed acids and alkalies in the 

 form of weak solutions of acetic acid and of caustic soda 

 or potash. In microscopic preparations granular matter is 

 often seen which really belongs to the tissues, but owing to 

 its form and size it cannot be easily distinguished from 

 spherical Bacteria ; the acids and alkalies have the power of 

 dissolving this matter, leaving the Bacteria unaffected. Koch 

 observed that this method was very useful in the case of 

 large Bacteria and Zooglaea masses, but it failed with the 

 more delicate specimens because the granular matter was 

 sometimes unaffected while the smaller Bacteria were 

 destroyed. Various stains were then experimented with, 

 among others Haematoxylin was tried, but it does not stain 

 rod shaped Bacteria at all, and the spherical forms only a 

 little, so that although it was useful, it was not satisfactory. 

 Weigert introduced the use of Aniline dyes using principally 

 Methyl-violet. Koch worked at this method, and found that 

 by using various Aniline dyes and by manipulating each 

 tissue in a way special to itself, he obtained much better 

 results. He came to the conclusion, however, that though 

 the method is good for large Bacteria, it is difficult for 

 smaller, and useless for the smallest, so that something more 

 was required. When taking micro-photographs of Bacteria 

 embedded in Canada Balsam, he observed that the image 

 consisted of a structure picture and a colour picture, and that 



