114 CLINICAL BACTERIOLOGY AND ILEMATOLOGY 



a secondary rash after puncture of the skin, or, according to 

 some authors, in 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 syphilis. It is difficult to 

 find in gummata or in any tertiary lesion. 



The lesion which has most usually to be examined in prac- 

 tical work is a supposed chancre. The results of this exam- 

 ination 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, and you will 

 usually find that drops of clear serum will collect on the sur- 

 face. 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 spirochsetes 

 can be demonstrated: (i) The use of dark background 

 illumination, or the so-called ultra-microscope; (2) Bum's 

 method of dark background illumination by means of Indian 

 ink; (3) various staining processes. 



i. Dark background illumination is certainly the best pro- 

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

 the technique 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 sur- 

 face 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 con- 

 ditions 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 illumination, such 

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



