36 BACTERIOLOGICAL DIAGNOSIS. 
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 bac- 
teria may be recognised 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 micro- 
scope 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 purpase, 
and put the microscope back into its case. If oil or 
balsam should get dried on the lens wipe it with a 
handkerchief just moistened with xylol or pure turpen- 
tine and then wipe quickly with a dry handkerchief. 
Never dip the point of a lens into xylol or alcohol. 
Never remove the front combination of an oil immersion 
lens for cleaning or any other purpose. 
