THE BA GTE RIO L OGICA L LA BORA TOR Y 1 7 



oil-immersion lenses, a drop of cedar oil is placed 

 on the front glass, the lens in use is then lowered 

 on to the slide until contact is made. The lens is 

 then focussed by the fine adjustment (Fig. SF) 

 until the object is seen sharply defined. 1 



It is desirable that the bacteriologist's microscope 

 should be fitted with a revolving triple nose-piece 

 (Fig. 3 A); by this means three objectives (of 

 different magnifying power) can be brought succes- 

 sively into position, without unscrewing. 



The eye-piece or ocular is also an essential part 

 of the microscope (see Fig. 3c); and for bacterio- 

 logical research, the whole (five in number) of Zeiss' 

 huyghenian eye-pieces are recommended. The fol- 

 lowing table shows the magnifications of Zeiss' 

 objectives and eye-pieces with a tube of 155 milli- 

 metres in length i.e. the Continental microscope 

 with a short tube : 



1 Zeiss no longer makes the ^ oil-immersion objective ; this 

 lens has now been superseded by the introduction of a new 

 series of objectives the apochromatic lenses made of the new 

 glass. These lenses are said to excel the ordinary objectives, 

 by giving almost perfect achromatism and sharpness of image 

 over the whole visual field (see Abbe's paper in Sitzungxberichte 

 d. med.-naturw. Gesettschaft zu Jena, 1886). 



