OBJECTIVES AND OCULARS. 
6l 
and friend, the “ fan of diffraction is closed up by 
immersion.” A very simple diagram will illustrate 
these points. 
A pencil emerging from the cover-glass A B into air 
may pass towards C, and the objective may not grasp 
it ; in water the direction may be towards D, in oil 
towards E, and evidently an objective may lose C and 
grasp D, or may lose C and D and grasp E. 
Water immersion objectives are not nearly so much 
used as oil immersions, or—as they are not quite accu¬ 
rately called sometimes—“ homogeneous ” immersion 
objectives. The practical advantages of the immersion 
system are numerous and grave. Much greater aper¬ 
ture can be obtained, more light passes through the 
lenses, greater “ working distance ” is possible. Work¬ 
ing distance is simply the distance between the front of 
the objective and the top of the cover glass when the 
object is in focus ; but it is of some importance, for we 
have known even immersion objectives with so short 
working distance that we have been unable to examine 
preparations which happened to have cover-glasses 
