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decreased and increased when lamp-light is used. It 

 should be exacted that the focus of the concave mirror 

 be within the limits of its adjustment. The serious 

 disadvantage of its incorrectness in this respect can 

 easily be seen by taking, for instance, a -^ objective 

 which will resolve P. angulatum nicely with central 

 light, when the mirror is exactly focused. By mov- 

 ing the latter out of focus it will be seen that the 

 objective loses in performance, and if this is carried 

 sufficiently far it will arrive at a point where the 

 objective will cease to show any lines. The effect will 

 be the same on any other object, and is caused by the 

 lack of proper concentration of light on the slide. 

 When oblique light is used, unless the diaphragm 

 moves with the mirror, it should be removed, as the 

 advantage of obliquity is diminished or destroyed by 

 the loss of light. 



The cover-glass exerts probably the greatest influ- 

 ence in testing as well as in general work. This 

 should be used of a thickness which corresponds to 

 that to which the objective (if non-adjustable) was 

 originally corrected. If thicker or thinner covers be 

 used, the objective will be spherically over or under 

 corrected, and will have to be moved correspondingly 

 above or below the plane ( outline ) of the object 

 to distinguish its structure, if the variation is con- 

 siderable the difference between the two planes will 

 be so great that it will cease to show any structure, 

 and it may then be said to be lacking in defining 

 power, although in reality it possesses it but is not 



