EXAMINATION OF FILMS USE OF MICROSCOPE 27 



A word of warning must be given as to the interpretation of Gram's 

 stain. Many organisms, if not all, that stain by Gram lose their 

 power when old and degenerate. In examining cultures, therefore, 

 do not draw any conclusions unless you are working with one that is 

 young and vigorous. The same thing happens, even more quickly, 

 when some bacteria are acted on by the defensive agencies of the 

 body, such as occur in serum, in the leucocytes, etc., so that if you 

 find Gram-negative bacteria in pus, and especially those lying in 

 leucocytes, do not be too ready to assume that they actually belong 

 to Gram-negative species. With care, no difficulty should arise, as 

 you will probably find some organisms which are Gram-positive, and 

 which are otherwise quite like the others, and intermediate forms, 

 partly stained and partly unstained. The other error, that of regard- 

 ing Gram-negative organisms as Gram-positive, can only arise from 

 bad technique especially perhaps from using a thick film, the central 

 parts of which are not easily decolorized. 



EXAMINATION OF FILMS USE OF MICROSCOPE 



Daylight is the best illttminant 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 paraffin 

 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 adjust- 

 ment) 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 examina- 

 tion 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-inch). Remember that 



