EXAMINATION OF FILMS — USE OF MICROSCOPE 27 



EXAMINATION OF FILMS— USE OF MICROSCOPE 



Daylight is the best illuminant for microscopic work, and 

 the light reflected from a white cloud opposite the sun is 

 best of all. Direct sunlight is useless, but the light obtained 

 from a ground-glass window on which the sun is shining is 

 very good. 



For work at night the light from an incandescent gas-burner at 

 a distance of 2 or 3 feet is excellent, but an ordinary parafhn 

 lamp will answer quite well. 



Having arranged for a suitable source of light, turn the flat 

 mirror uppermost, and move it about until 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 ^ inch). Remember that the 

 "working distance" of all lenses is necessarily less than their 

 focal distance, and that a ^-inch lens focusses at a distance from 

 the object which is decidedly less than |- 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 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 downwards on 

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



