7 2 BACTERIOLOGY. 



width, and one inch in depth. It may be easily 

 constructed by cementing together two slips of glass 

 to form the back and front, with three slips of stout 

 glass with ground edges forming the sides and 

 base. The front may be constructed of thin glass, 

 and the base of the vessel made to slope so that the 

 test-tube when placed in the trough has a tendency 

 to be near the front. The trough is filled with a 

 mixture of the same refractive index as the nutrient 

 gelatine. The latter has a refractive index rather 

 higher than water, which is about 1*333 ; alcohol 

 has a refractive index of 1*374. The trough is 

 filled with water, and alcohol is then added until the 

 proper density is reached. The test-tube is placed 

 in the trough, and held in position by means of a 

 clip. The trough can be fixed on the inclined stage 

 of the microscope, and the contents of the tube 

 conveniently examined with low power objectives. 



PL ATE- CULTIVATIONS. 



The key to the success of Koch's method of 

 cultivation on solid media consists in the employ- 

 ment of plate-cultivations. By this means, as 

 has already been mentioned (p. 63), a mixture of 

 bacteria, whether it be in fluids, excreta, or in 

 artificial cultivations, can be so treated that the 

 different species are isolated one from the other, 

 and perfectly pure cultivations of each of the 

 cultivable bacteria in the original mixture esta- 

 blished in various nutrient media. We are enabled 



