92 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



fluid from a blister raised on or near a lesion of such a rash. It 

 appears to be especially abundant in the pemphigoid rashes of 

 hereditary syphiUs. It is difficult to find in gummata or in any 

 tertiary lesion. 



The lesion which has most usually to be examined in practical 

 work is a supposed chancre. The results of this examination are 

 often of enormous value, especially since the introduction of " 606," 

 since the early recognition of the nature of the disease may lead 

 to early abortive treatment, so that no later symptoms of any sort 

 ever develop. The 'method of collecting the material is of prime 

 importance. Cleanse the surface of the lesion of all pus and 

 secretion, using normal saline solution, methylated spirit, or plain 

 water ; then dry it carefully, using a piece of lint or cotton-wool, 

 aiid you will usually find that drops of clear serum will collect 

 on the surface. The process may be hastened by gently squeezing 

 the edges of the lesion. This material is very much richer in 

 spirochaetes than the ordinary secretion, in which they are largely 

 digested by the juices, bacterial enzymes, etc. 



There are three chief methods by which the spirochaetes can be 

 demonstrated: (i) The use of dark background illumination, or 

 the so-called ultra-microscope ; (2) Burri's method of dark back- 

 ground illumination by means of Indian ink ; (3) various stain- 

 ing processes. 



1. Dark background illumination is certainly the best process 

 where much work of the sort has to be done, and, when the tech- 

 nique is mastered, its use enables a diagnosis to be made more 

 quickly and certainly than do the other processes. The theory of 

 the method is simple. A ray of light passing through a specimen 

 parallel, or almost parallel, to the surface of the slide will not 

 enter the objective, and the field will remain absolutely dark. 

 If, however, this ray meets with any object, however small, it 

 will be reflected in all directions, and a part will enter the lens, 

 so that the object is seen as a brilliant spot on a black ground. 

 In practice the optical conditions are secured by means of a 

 special condenser and a collar which fits inside the oil-immersion 

 lens. The light has to be parallelized by means of a condensing 

 lens before it reaches the mirror, and a brilliant source of illumina- 

 tion, such as an arc-light or Nernst lamp, is advisable, though 

 hardly necessary. Full instructions are provided with the 

 apparatus, which can be obtained from the usual agents. 



2. Burri's method is probably the most suitable for the prac- 



