MICROSCOPIC EXAMINATION OF PREPARATIONS 207 



Between the condenser and reflector is placed the "iris" 

 diaphragm, the aperature of which can be regulated, as 

 circumstances require, to permit of either a very small or 

 a very large amount of light passing to the object. 



The nose-piece (i) consists of a collar, or group of collars 

 joined together (two or more), that is attached to the distal 

 end of the tube of the microscope. It enables one to attach 

 several objectives to the instrument in such a way that by 

 simply rotating the nose-piece the various lenses of different 

 power may be conveniently used in succession. 



Dark-field Illumination. This refers to a result obtained 

 through the use of an apparatus that so deflects and reflects 

 the light's rays that the field is dark and the objects in it 

 brilliantly light. It is used only for the examination of 

 unstained living objects and is capable of revealing the 

 most minute particles and microorganisms. It is especially 

 useful for the study of the normal morphology and move- 

 ment of spirochete and trypanosomes and for the detection 

 of bodies so small or otherwise so constituted as not to be 

 visible by the ordinary methods of microscopic examination. 

 Two forms of the illuminator are in use one that slips into 

 the collar ordinarily carrying the sub-stage condensing 

 apparatus, the other is made in the form of a slide and is 

 placed on the stage directly over the opening for illumina- 

 tion. Both provide for the complete cutting off of direct 

 central rays of light, allowing only the lateral rays to reach 

 the objects and be reflected by them to the eye. Both require 

 very intense illumination for the best results. This may be 

 obtained from a Welsbach burner, or a small arc or incan- 

 descent light. In both cases the light's rays must be 

 condensed upon the reflector of the microscope by means 

 of a condensing lens. 



