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. 

 ,ook 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 J in.). Remember 

 that the "working distance " of all lenses is necessarily 

 less than their focal distance, and that a J 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 



D 2 



