70 BACTERIOLOGY. 



After experimenting with different liquids — solutions of salts, 

 and various essential oils — Abbe recommended cedar oil as most 

 suitable for the purpose. In its optical properties it very closely 

 resembles crown glass, and it is far more convenient for use than 

 any watery solutions of salts, especially when it is necessary to 

 make a more or less prolonged examination of an object. 



The difference between the dry, water, and oil immersion systems 

 may be illustrated, as Frankel has pointed out, by a v6ry simple 

 experiment. If a glass rod is inserted into an empty test-tube, it 

 is easily visible owing to the difference in refraction between the 

 glass and the surrounding air. If the tube is filled up with 

 water the rod is seen with difficulty, and if, instead of water, cedar 

 oil is used, the part of the rod immersed in the oil will entirely 

 disappear from view. The rays of light pass through an optically 

 uniform medium in the experiment with cedar oil, and no refraction 

 or reflection of rays of light can occur. 



To use an oil immersion objective, a minute drop of cedar oil 

 is placed on the centre of the cover-glass, and the lens lowered 

 by means of the coarse adjustment until it touches the oil. The 

 specimen is then carefully brought into focus with the fine adjust- 

 ment. If the slide is held between the finger and thumb of one 

 hand, and moved from side to side while the other hand is working 

 the fine adjustment, there can be no danger of injuring either the 

 objective or the specimen. 



Microscopes are made upon either the Ross or the Jackson 

 model. In the Ross model the body of the microscope is fixed 

 at its base to a transverse arm, which is raised or lowered with 

 it by the rack and pinion. In the Jackson model the body is 

 supported for a great part of its length on a solid " limb." 



In the Ross model, unless the body and transverse arm are very 

 solid as in Powell and Lealand's microscopes (Fig. 23), there will be 

 vibration at the ocular end ; but in the Jackson model vibration is 

 practically prevented, and this is most essential, especially in working 

 with very high powers. 



The steadiness of the microscope also largely depends upon the 

 form of stand. There are four different types of stands. The 

 tripod (Fig. 23) ; the plate, with double columns ; the single column, 

 ending in a plate or a bent claw; and the horse-shoe (Fig. 18). 



The tripod stand with cork feet is the steadiest form of stand, 

 but it is cumbrous and expensive, and these objections also apply to 

 the model made by Ross. 



The single upright should be unquestionably condemned, as it 



