50 EXPERIMENTAL DAIRY BACTERIOLOGY 



ring of vaseline; press down on the cover. The, vaseline should 

 form a perfect seal, thus preventing evaporation and enabling 

 a small drop to be kept under observation for hours. On ac- 

 count of the transparency of the bacteria focusing is often 

 difficult. The drop should be placed in the optical axis of the 

 microscope, which is easily done by lowering the tube with 

 the 4-mm. objective in place within one eighth of an inch 

 of the cover glass, and then bringing the drop under the lens. 

 With the eye at the side, lower the tube until the objective is 

 nearly in contact with the cover. Close the diaphragm until 

 the field is light gray in color, and focus very slowly upward. 

 After finding the organisms the slide should be moved until 



FIG. 12. A HANGING-DROP PREPARATION 



the edge of the drop is found, as here the organisms can be 

 observed to better advantage than in the deeper portion of 

 the drop. The oil-immersion lens may then be used if de- 

 sirable. The use of artificial light renders the organisms 

 more easily visible. 



The form, size, and grouping of the cells should be noted, 

 the power of independent motion, the nature of the motion, 

 direct, sinuous, rotating, etc. The molecular or Brownian 

 motion (pedesis) must not be confounded with vital motion. 

 This molecular motion is a property common to all finely 

 divided substances suspended in liquids, and consists of a 

 vibratory or trembling rather than a progressive motion. 

 In order that an organism be classed as motile, the relative 

 positions of the cell with reference to other cells must change. 



Exercise. Examine in hanging drop the cultures of yeast and of 

 bacteria furnished you, and also a preparation made by suspending 

 a little India ink in water. 



