come together at some point above, and the rays from 

 the surface being contained within a comparatively 

 small space, cause an increased intensity. This point 

 is called 'the focal point and usually coincides with the 

 opening of the stage when parallel rays such as from 

 the sky are used. When the light comes from a source 

 considerably nearer to the mirror, as from a lamp, and 

 the rays are diverging, the focal distance becomes con- 

 siderably Ignger. Some of the intensity is lost in 

 consequence as well as the degree of convergence. 

 For this reason some mirror-bars are so arranged that 

 the distance of the mirror from the stage may be varied 

 to accommodate the variation in the location of the 

 source of light. While this is of considerable aid, 

 there is in some instruments not sufficient room for a 

 complete accommodation, with the result that, under 

 certain conditions, the utmost effectiveness of the 

 microscope is not obtained. 



Substage Diaphragm. This is provided for 

 regulating the amount of light. While it should be 

 possible to use the mirror at its utmost capacity, it very 

 often occurs that for certain investigations a profusion 

 of light is more harmful than otherwise. When there is 

 too much light objects are said to be drowned in it, and 

 this often makes it impossible to determine structure. 

 An intelligent use of the diaphragm is of great service. 



The revolving diaphragm is the most simple and 

 consists of a black disk which rotates on a pivot and 



53 ' 



