13 



Place a small drop of water in the center of a clean 

 cover glass. With a sterilized and cooled platinum wire, 

 which is fused into the end of a glass rod, 6 or 7 inches 

 long, transfer a minute amount of the bacterial growth 

 from the surface of one of the potato cultures, to this drop 

 of water. Place the cover-glass on a table or on a 

 block. Select a clean " concave slide " and apply a ring 

 of vaseline with a brush or stick to the edge of the 

 cavity. Invert the slide thus prepared over the cover- 

 glass with the drop and gently press down till the little 

 chamber is sealed air tight. The slide is now ready for 

 microscopic examination, and if properly made the drop 

 of water will be flat, and will not run when the slide is 

 placed on its edge. 



Place the slide, with the cover-glass uppermost, on 

 the stage of the microscope, .and first find the edge of the 

 drop with a low power No. 3 objective. If too much 

 light is present, constrict the diaphragm. The edge of the 

 drop should be seen as a sharp line passing through the 

 field of the microscope. Holding the slide between the 

 thumb and forefinger of the left hand, slowly move it so 

 that the edge of the drop constantly remains in the field. 

 In this way the entire edge or circumference of the drop 

 should be examined, chiefly for the purpose of practice in 

 moving a slide under the microscope. Owing to the min- 

 ute size of bacteria they cannot be seen under this mag- 

 nification. To observe the individual cells, therefore, 

 recourse must be had to the higher powers No. 7 objec- 

 tive, or the -iV inch homogeneous oil-immersion objective. 



Examination with No. 7 objective. Having found the 

 edge of the drop with No. 3 objective, replace this by No. 

 7, by rotating the nose-piece. Then lower the tube of the 

 microscope by the coarse adjustment till the objective 

 almost touches the cover-glass. The field of the micro- 

 scope now is usually very dark, hence open the diaphragm 

 a trifle to admit enough light to see distinctly. With the 

 fine adjustment now raise the microscope tube till the 

 edge of the drop is brought out distinctly. By focussing 



