ISOLATION OF BACTERIA IN PURE CULTURE 545 



scratcliing be performed in a drop of immersion-oil. In an 

 objective (preferably a different one from that with which the 

 growth is observed, as the micrometer lines will disturb observa- 

 tion) is placed a squared eyepiece micrometer, which is cemented 

 on to prevent displacement. 



If we now focus sharply on the scratches on the slide with the 

 low power of the microscope, the lines in the eyepiece micrometer 

 will be intersected by these scratches in a quite specific manner 

 (cf. fig. 3), and thus we obtain two distinguishing marks instead 

 of one. The course of procedure will now be as follows: the 

 agar is placed on the scratched area of the slide, and we search 

 for a place where there is only one organism within the field of 

 vision. The spot is marked by means of the scales of the me- 

 chanical stage, the objective with the attached micrometer is 

 placed in the tube, the microscope is adjusted to low magnifica- 

 tion and focussed sharply on the scratches. Careful drawings 

 are made on squared paper of the position of these scratches 

 in relation to the eyepiece micrometer, the slide being now placed 

 in a sterile Petri dish with a piece of moist filter paper at the 

 bottom. (The filter paper must not be too wet as this may cause 

 the development of so much aqueous vapour during incuba- 

 tion that the glasses become wet enough for the agar squares 

 to sUde, when the whole experiment is ruined.) By means of 

 thus marking the position of the organism we have always 

 succeeded in hitting upon exactly the same spot for repeated 

 examination. 



The growth is now watched at proper intervals, the adjust- 

 ment being performed so that, firstly, the scales of the mechani- 

 cal stage are placed in the proper mutual relation, which we have 

 noted do^NTi, secondly, with the low power of the microscope we 

 make the scratches on the slide correspond to the proper points 

 in the eyepiece micrometer, and, finally, eyepiece and objective 

 are changed, and we can now easilj^ observe the alterations in 

 the small colony in development. (The slides must of course 

 previously be cooled to the same temperature as that of the 

 objective, in case examination takes place at a lower tempera- 

 ture than that of incubation, as, othel•^^'ise, condensed moisture 



