VACCINE-THERAPY 7 



case he should remove the eyepiece and look down the 

 tube, moving the mirror until he sees clearly the reflected 

 light. 



We will now presume that a slide has to be examined 

 under a ^ oil immersion. If the specimen is only faintly 

 stained, it is well to make a ring with a grease pencil round 

 the field. This serves as a border from which to focus, and 

 is most useful. A drop of cedar-oil should be placed on 

 the specimen, and the tube lowered by the coarse adjust- 

 ment until the objective almost touches the slide. The 

 beginner should now place his eye on a plane with the 

 stage, at the same time steadily lowering the tube. 

 Immediately the lens touches the oil it will appear to rise 

 to meet the objective. With the fine adjustment then 

 carefully focus until the objects on the field become well 

 defined. 



If the specimen contains bacteria in irregular distribu- 

 tion, it is advisable to make one's examination by way of 

 locating the best area with a low power first ; in fact, 

 many bacteriologists carry out the process as a routine 

 practice. It may be added an oil immersion lens transmits 

 two or three times more light than a dry lens of the same 

 power, while the Abbe condenser collects the rays of light 

 reflected from the mirror, and concentrates them at a 

 point about 2 millimetres above its upper surface. 



If the nozzle of the objective requires cleaning — and this 

 should be done fairly often — the oil can be removed by 

 xylol, turpentine, or benzine on a piece of chamois leather. 



The whole instrument should be kept free from dust and 

 other accumulations, and when not in use should be covered 

 with a glass shade. 



The Incubator and Incubation. 



To grow bacteria outside the animal body some form of 

 incubator is required. This consists of a metal box with 

 double walls, the space between which is filled with water, 

 and the whole encased in some non-conductor, such as felt 



