MICROSCOPE FOR STAINED BACTERIAL PREPARATIONS 93 



suspended (water, bouillon, milk, wine, pus, etc.). These slight 

 differences would be lost in a flood of light and the iris diaphragm 

 should be closed considerably. It is much easier for the beginner 

 to examine stained preparations of bacteria than the unstained ones, 

 hence the steps in the former procedure will be given first. 



Steps in Using the Microscope for Stained Bacterial Preparations. 

 1. Place the instrument in front of the source of light in a vertical 

 (not inclined) position and deposit the preparation on the stage of 

 the microscope. See that the -surfaces of the objectives and eye- 

 pieces are free from dust and grease and otherwise clean. 



2. Bring the f-inch low-power lens into the centre and lower 

 the tube on the coarse adjustment so that it is about one inch from 

 the cover-glass. 



3. Illuminate the object by manipulating the plane reflector (do 

 not use the concave mirror). Have iris diaphragm wide open and 

 slowly lower the tube on the coarse adjustment until the color of the 

 object can be well recognized. Now, again manipulate the reflector 

 and lower the Abbe condenser until the field of vision is well and 

 uniformly illuminated and until no shadows of the window frame 

 or parts of the lamp are seen in the field of vision. 



4. Move the microscopic slide around until a place is found 

 where the stain is neither too dense nor too scanty. Place this spot 

 in the centre of the field and clamp down the slide so that it is immov- 

 able. Look again to see whether the selected spot is still exactly in 

 the centre of the field; if not, move the slide under the clamps until 

 the desired spot is where it is wanted. 



5. Raise the tube somewhat on the coarse adjustment and swing 

 the y^inch oil-immersion lens into the centre. Place a drop of cedar 

 immersion oil on the centre of the cover-glass; run the immersion 

 lens on the coarse adjustment into the oil. Raise the tube on the 

 coarse adjustment so that the drop of cedar oil is drawn out. The 

 oil-immersion lens is now considerably above the proper focal dis- 

 tance of the object. 



6. Raise the substage condenser as high as it will go. (Its front 

 lens is now on a level with the stage of the microscope.) Again, man- 

 ipulate the plane mirror so that the field is well and uniformly lighted. 

 Now, slowly, with the coarse adjustment, lower the tube under the 

 guidance of the eye which watches the field of vision through the 

 ocular. As soon as some color is observed the fine adjustment or 

 micrometer screw must be used until a clear image is obtained; that 

 is, until the oil-immersion lens is sharply in focus. 



The microscopic picture so obtained may sometimes still be im- 

 proved by lowering the substage condenser a very little bit, and by 

 closing the iris diaphragm somewhat; but the beginner should be 

 careful in attempting these corrections, which require a good deal 

 of skill and judgment in the interpretation of the image. 



The danger for the beginner in the use of the oil-immersion lens 



