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instance from a lamp and the rays are diverging, the 

 focal distance becomes considerably longer, and when 

 very close may be twice as long. Some of the intensity 

 is lost in consequence, as well as the degree of con- 

 vergence. For this reason mirror-bars are so arranged 

 that the distance of the mirror from the stage may be 

 varied to accommodate the variation in the source of 

 light. While this is of considerable aid, there is not 

 sufficient room for a complete accommodation, with 

 the result that, under certain conditions, the utmost 

 effectiveness of the microscope is not obtained. 



Diaphragm. This is provided for regulating the 

 amount of light. While the mirror should work to 

 its utmost capacity, it very often occurs that for cer- 

 tain investigations a profuseness of light is more 

 harmful than otherwise. When too much light exists, 

 objects are said to be drowned in it, and often makes 

 it impossible to determine structures. An intelligent 

 use of the diaphragm is of great service. 



Besides the revolving diaphragm there are a num- 

 ber of other forms which may be said to be better 

 for instance, the so-called cup diaphragms, which 

 require a separate piece for each aperture and which 

 are held by a special sub-stage receiver. Then the 

 dome diaphragm, which is a new application of the 

 ordinary revolving diaphragm. It consists of a sub- 

 stage fitting having a dome to which is fitted a 

 curved revolving diaphragm 



The ideal regulator of light is the Iris diaphragm 

 consisting of a series of overlapping blades placed 



