EXAMINATION OF FILMS. 35 
a beam of light is thrown on to the condenser. 
Remember :— 
In examining stained specimens use a large diaphragm. 
In examining unstained objects use a small diaphragm. 
-You are now about to examine a stained specimen. 
Place the slide on the stage, putting the stained film in 
the centre of the aperture, and turn on the low power. 
Look down the eye-piece and move the mirror about 
until the field is brilliantly illuminated. Focus the 
microscope (using the coarse adjustment) until the 
image is clearly defined. Now move the slide about 
until there is a deeply stained area in the centre of the 
field. This area will not necessarily be the best for 
examination with a higher power, but it will serve to 
catch the eye when focussing the lenses which focus at 
a short distance from the object. 
Now turn on the high power (the $in.). Remember 
that the ‘working distance” of all lenses is necessarily 
less than their focal distance, and that a } in. lens 
focusses at a distance from the object which is decidedly 
less than one-sixth of an inch; so also with the other 
powers. Lower the lens until it almost touches the 
object and screw up the sub-stage condenser as high as 
it will go. Look down the microscope and focus 
slightly upwards, 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 down- 
wards on to the film, keeping a sharp look out for the 
first appearance of colour, but for beginners the fore- 
going 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 
D2 
