78 SELECTION AND USE 



in the hands of experienced microscopists, but beginners will 

 find it difficult to use them. Objectives of very high angular 

 aperture have in general very short working distances, but there 

 are great differences in this respect amongst the products of 

 different makers. Working distance does not depend upon 

 angular aperture alone. 



Immersion and " Homogeneous " Lenses. Ob- 

 jectives which require a drop of liquid between the front lens 

 and the covering-glass of the object, are now in common use, 

 and have been deservedly received with general favor. The 

 liquid employed serves two important purposes. In the first 

 place, it partially extinguishes two of the glass surfaces (the 

 front surface of the objective and the upper surface of the 

 covering-glass), and thus it prevents, to a considerable extent, 

 the loss of light which always occurs at these surfaces ; and in 



Fig, 20. 



the second place, it enables the objective to gather up rays 

 which otherwise would pass outside of it. In the ordinary im- 

 mersion objective, water is the liquid which is used, and its 

 action will be readily understood from an inspection of the 

 accompanying diagram, where c is the front of the objective ; 

 Y is the cover-glass ; E the space between the front lens and 



