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EXPERIMENTAL DAIRY BACTERIOLOGY 



slightly with the fingers. With high-power lenses never focus 

 down with the eye at the ocular. 



Oil-immersion objective. The rays of light passing obliquely 

 through glass into air are refracted away from the perpen- 

 dicular ; or, in other words, the rays of light entering the 

 lens of the microscope are less in number than if there were 

 no difference between the indices of refraction of glass and 



EIG. 10. PRINCIPLE OF THE OIL-!MMERSION LENS 

 L, lens ; 2), object slide ; Od and Ob, rays of light. (After Hager-Mez) 



air. By placing a liquid having the same index of refraction 

 as glass between the lens and the object, the amount of 

 light entering the lens is increased, and as far as the amount 

 of light is concerned, the object, the slide, the lens, and the 

 intervening liquid act as a single piece of glass. The immer- 

 sion liquid used is cedar oil, of such a degree of concentration 

 as to have the same index of refraction as glass. If the oil 

 becomes thickened by evaporation of the solvent, the bottle 

 should be cleaned and a fresh supply obtained. 



