ISOLATION OF BACTERIA IN" PURE CULTURE 547 



be almost inconceivable ill-luck if the "alien" microorganism 

 should be one that was closely related to the isolated one. 



Now, the new-formed colony having reached a convenient 

 size, sub-cultures should be prepared from it. At this point 

 the colonies have grown so large as to be distinctly visible with 

 the low power of the microscope, the time and mode of re-inocu- 

 lation depending on the relative situation of the colonies. In 

 case the colonj^ the shape of which we recognize with the low 

 power of the microscope, is placed in a sufficiently isolated 

 position, we may defer inoculation until it has grown big enough 

 to allow of our conveniently inoculating from it by means of a 

 fine inoculation needle under the microscope at low magnifica- 

 tion. If there is any danger of neighbouring colonies impinging 

 upon it, we must undertake inoculation while it is yet smaU. 

 As mentioned, Hort used a special apparatus for this purpose. 

 A small harpoon, which one may prepare oneself, will how- 

 ever do. 



On the front lens of an objective is placed a small lump of 

 modelling wax to which is attached a fine thin platinum wire 

 not thicker than 0.15 mm. with a blunt end (cf. fig. 4). Previous 

 to "harpooning" the colony some preparatory practice is nec- 

 essary. A small agar square is excised and fitted in the usual 

 way and placed on a slide. A droplet of India ink or some 

 other staining fluid is deposited on the agar with a mapping- 

 pen. This spot is now centered in the field of vision at the low 

 power of the microscope, and the objective ^ith the attached 

 platinum needle, which is screwed on to the nose-piece, is directed 

 across the spot, the needle being adjusted by a pressure from 

 the fingers so as to be mounted exactly above the spot, and 

 gently depressed so as to touch the agar. The point of contact 

 is readily discerned with the low power of the microscope and 

 marked by means of the eyepiece micrometer. We know now 

 exactly where the needle will hit, being able then to inoculate 

 from the colony by adjusting it to the exact point in relation 

 to the eyepiece micrometer at which the needle hit last. The 

 needle is carefully lowered into the colony until it touches the 

 agar plate. The point of contact is most readily noticed by 



