190 BACTERIOLOGY 



Dark-field Illumination. — ^This refers to a result obtained 

 through the use of an apparatus that so deflects and reflects 

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

 brilliantly light. It is used only for the examination of 

 unstained objects and is capable of revealing the most 

 minute particles and micro-organisms. It is especially 

 useful for the study of the normal morphology and move- 

 ment of spirochete 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 substage condensing apparatus, the 

 other is made in the form of a slide and is placed on the 

 stage directly over the opening for illumination. 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 light. In 

 both cases the light's rays must be condensed upon the 

 reflector of the microscope by means of a condensing lens. 



Microscopic Examination of Cover-slips. — The stained cover- 

 slip is to be examined with the oil-immersion objective, 

 and with the diaphragm of the sub-stage condensing appara- 

 tus open to its full extent. The object gained by allowing 

 the light to enter in such a large volume is that the contrast 

 produced by the colored bacteria in the brightly illuminated 

 field is much more conspicuous than when a smaller amount 

 of light is thrown upon them. This is true not only for 

 stained bacteria on cover-slips, but likewise for their differen- 

 tiation from surrounding objects when they are located in 

 tissues. With unstained bacteria and tissues, on the con- 



