70 



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 certain investigations a profuseness of 

 light is more harmful than otherwise. When too 

 much light exists, objects are said to be drowned 

 in it and thus often makes it impossible to 

 determine structure. An intelligent use of the 

 diaphragm is of great service. 



Besides the revolving diaphragm there are other 

 forms which maybe said to be better for instance, 

 the so-called cap diaphragms, which require a sep- 

 arate piece for each aperature and which are held 

 by a special substage receiver; then the dome 

 diaphragm, which is a new application of the ordi- 

 nary 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 

 around a central opening, size of which may be 

 varied by means of a lever or milled edge. In the 

 ordinary revolving form the aperture is of neces- 

 sity at some distance from the object and does not 

 fully control the light on account of the stray rays, 

 which the other three forms accomplish. The 



