MICROSCOPIC EXAMINATION ' AND STAINING; ^l^frliODS. 101 



with the opening F'F. The rays of light are focused upon the ob- 

 ject to be examined by the mirror or Abbe condenser. Those 

 rays of light, such as BN, that strike the glass perpendicularly 

 pass through and enter the lens without any deflection. A ray of 

 light, such as AB, striking the glass at a considerable angle, is 

 refracted upward and toward the normal or in the direction of BD. 

 Upon entering the air it would again be refracted and leave the 

 glass in a direction parallel to the original ray, or DE, and would 

 not enter the lens. If, on the other hand, a drop of oil having the 

 same refractive index as the glass intervenes, there will be 

 no refraction at D, but the ray will pass through to the opening 

 of the lens. This is represented by the ray A'BD'F'. The use 

 of the oil, therefore, results in a more brilliantly illuminated field 

 and a clearer definition of the objects to be examined. 



Measuring Bacteria. Bacteria may be measured under the 

 microscope in one of several ways. A micrometer scale ruled on 

 glass may be inserted in the ocular, and the distance between the 

 lines determined by examination of a micrometer scale ruled upon 

 the slide or cover-glass examined under the microscope. When 

 the calibration has been effected, the ocular micrometer may be 

 used to measure the bacteria directly. The unit of microscopic 

 measure is the micron, the one-thousandth part of a millimeter. 



Examination of Living Bacteria. Hanging Drops. The deter- 

 mination of the motility of bacteria can best be accomplished by 

 the examination of the living cells under the microscope. A hang- 

 ing-drop preparation is commonly used for the purpose. A loop- 

 ful of broth culture of the organism to be tested is placed upon the 

 center of a carefully cleansed and flamed cover-glass. Growth 

 from an agar or other culture may be used by substituting a 

 drop of physiological salt solution or sterile bouillon and introduc- 

 ing a minute quantity of the growth on a platinum needle. This 

 drop is then carefully inverted over the cavity in a hollow ground 

 slide, and sealed with a little vaselin. It may be examined with 

 a high-power dry lens or with the oil-immersion objective. The 

 drop may most easily be brought into focus at its margin. The 

 light must be carefully regulated by means of the mirror and the 

 iris diaphragm of the Abbe condenser to make the bacteria most 

 clearly visible. 



