28 CLINICAL BACTERIOLOGY AND H/EMATOLOGY 



the "working- distance" of all lenses is necessarily less than 

 their focal length, and that a J-inch lens focusses at a distance 

 from the object which is decidedly less than J inch; so also 

 with the other powers. Lower the lens until it almost touches 

 the object, and screw up the substage condenser as high as 

 it will go. Look down the microscope and focus slightly up- 

 wards, using the coarse adjustment, until you catch a glimpse 

 of colour; then focus very slowly until the object is sharply 

 defined. 



After a little practice you will be able to focus downwards 

 on to the film, keeping a sharp look-out for the first appear- 

 ance of colour, but for beginners the foregoing method is 

 easier and safer. 



Study the object with the high power, and move it about 

 until you find an area where the bacteria are neither too thickly 

 nor too thinly spread, and are well stained. Make out as much 

 of their appearance as you are able to do with this power. Very 

 much can be done ; tubercle bacilli, gonococci, and many other 

 bacteria, may be recognized with this power, and the peculiar 

 arrangement of diphtheria bacilli can be seen. 



Apply the clips to keep the slide in place. 



Now raise the tube of the microscope for a short distance, 

 using the coarse adjustment, and place a small drop of cedar 

 oil on the centre of the cover-glass. Lower the tube (using 

 the coarse adjustment) until the nozzle of the lens touches the 

 drop of oil; then put your head on a level with the stage, and 

 continue to focus downwards, going very carefully, until the 

 lens almost touches the cover-glass. Next look down the 

 microscope, and focus upwards, using the fine adjustment, 

 until you begin to see colour; then go more slowly until the 

 film is well defined. 



Beginners are strongly urged to adopt this method of 

 focussing an oil-immersion lens until they have acquired a 

 considerable amount of practice. It takes a little time, but 

 this is well repaid by the absence of all danger or injury to 

 lens and cover-glass. After a time you may lower the lens 

 until it touches the oil, and then look down the microscope 

 and continue to lower it with the fine adjustment. 



After use, wipe the front of the immersion lens with a soft 

 silk handkerchief kept specially for the purpose, and put the 

 microscope back into its case. If oil or balsam should get 



