r. n. ATTFIELD — THE LOWER MICRO-ORGANISMS. 15 



material is washed oil and the preparation again dried and mounted 

 iu Canada-balsam. The various micro-organisms are thus lixed and 

 coloured, and hence may easily be seen. A somewhat special 

 microscope is necessary for their examination ; it must have what 

 is known as a one-twelfth-inch oil-immersion lens, and also must 

 carry a good substage condenser. Any less powerful objective is 

 almost useless for obseiwing these microbes, the largest of which 

 are only what is known in microscopical nomenclature as 5;«, 

 that is, five micromillimeti'es in diameter. A micromillimetre or 

 Greek m is the one- thousandth of a millimetre, and a yard is nearly 

 1000 millimetres. To make these mathematics less troublesome to 

 grasp, I may say that if the largest of the microbes were placed 

 side by side, over 5000 of them would be necessary to give a line 

 one inch long. To occupy the same space about fifty times as 

 many, that is, over a quarter of a million, of the smallest would 

 be required. 



A few words now as to the methods employed in bacteriological 

 research and analysis, and as to the principles on which are 

 established the causal, or at any rate intimate relation of micro- 

 organisms with disease in its various fonns. As a typical case, the 

 procedure in the bacteriological examination of water may be 

 taken. The first object is to ascertain the number of bacteria 

 present, and then to isolate and identify the various species. With 

 these objects in view, plate-cultivations are made. Into a tube of 

 sterile melted nutrient jelly, a small quantity of water, say one or 

 two cubic centimetres, is introduced. The water and jelly are 

 thoroughly mixed so as to equally distribute the micro-organisms 

 through the melted jelly. This is now poured out on to a glass 

 plate — hence the term "plate-cultivation" — and placed in an 

 apparatus to set. "When cold the plate is transferred to a moist 

 chamber and placed in the incubator, which is kept at a certain 

 temperature. In the course of from 12 to 36 hours a number of 

 little white or faintly-coloured spots are seen on the surface of the 

 gelatine. Each mici'obe in the water has set to work, and by rapid 

 reproduction has given rise to a colony of its own species. The 

 colonies may now be counted, and the approximate number of mi- 

 crobes in the original water be so ascertained. While these colonies 

 are small they can be easily picked up with a platinum needle, and 

 cover-glass preparations may then be made fi'om them. When a 

 tube of sterile jelly is inoculated with one of these little colonies, a 

 pure culture is obtained. This can be recultivated and examined, 

 when its various characteristics are recognised, and its possible 

 identity with a known species established. Before we can say that 

 a certain micro-organism is actually the causa causatts of any 

 particular disease it must respond to Koch's four postulates : («) 

 The micro-organism must be found in the blood, lymph, or diseased 

 tissues of man, or other animal, suffering from, or dead from, the 

 disease. (3.) The micro-organism must be isolated fi'om the blood, 

 lymph, or tissues, and cultivated in suitable media outside the 

 animal body, and these pure cultivations must be carried on 



