12 MEDICAL BACTERIOLOGY 



surface of it brightly and uniformly illuminated, at the same time the image 

 of the source of light appears clearly denned on the mirror. 



The dark field must be accurately centered by moving the screws on the 

 side of it, while looking through a low-power objective (without eyepiece in 

 tube) until the center of the field is brightly illuminated and all shadows and 

 haloes disappear. 



When proper adjustment has been achieved, the exact location of lamp, 

 condenser and microscope on the table should be noted. 



Though not absolutely necessary, it is much more satisfactory to make 

 observations with this microscope in a dark room than elsewhere. 



Only slides and cover glasses of the thickness for which the apparatus has 

 been constructed should be used. 



PREPARATION OF SPECIMENS FOR EXAMINATION 



In producing fluid or scrapings for examination for treponema pallidum, 

 it is important to avoid bleeding and to obtain the specimen free of blood. 

 Fluid can be obtained from macules and papules by gently pinching them with 

 a hemostat and nicking the elevated surface with a sharp knife. If done care- 

 fully, several drops of clear serum, free of blood cells, can be expressed. Speci- 

 mens can similarly be obtained from indurated surfaces and the margins of 

 ulcers. 



Lesions should not be washed or dressed with any antiseptic for at least 

 24 hours before specimens are obtained. About 6 hours before obtaining the 

 fluid they should be gently cleansed with normal salt solution and covered with 

 a gauze dressing, moistened with salt solution, which dressing is not disturbed 

 until the material for examination is collected. 



From macules, papules and ulcers, material for examination can be collected 

 and handled most easily and safely with straight, capillary glass tubes, about 

 6 inches long (Wright's tubes). Fluid from enlarged glands is withdrawn with 

 a syringe and needle. 



Examination of such specimens should be made as soon as possible never 

 more than an hour after they are collected. 



A drop of warm, sterile, salt solution is placed on the center of each of a 

 number of perfectly clean slides, and a drop of the fluid or scrapings to be ex- 

 amined is mixed with it. The mixture should form a thin film, free of air 

 bubbles, and occupy the entire space between the slide and the cover glass that 

 is dropped upon it. 



The preparations are luted with melted vaseline or paraffin, a drop of im- 

 mersion oil is placed on the dark-field condenser and the slide placed upon it, 

 another drop of immersion oil is placed on the cover glass and the oil immersion 

 lens brought into contact with it. The preparation is now ready to examine 

 when brought into focus. 



SLIDES AND COVER GLASSES 



Slides and cover glasses when purchased are seldom, if ever, clean enough 

 for use. The easiest and most satisfactory method of cleaning is to immerse 



