LESSON VII. 



STUDY OF INDIVIDUAL BACTERIA, THEIR PHYSICAL 

 AND CHEMICAL CHARACTERISTICS. 



For the examination of individual bacteria one should make use of a good micro- 

 scope fitted not only with the ordinary low-magnifying powers (50 to 500 diameters), 

 but also with a clear, flat objective giving with the usual oculars and elongation of the 

 tube an amplification of seven hundred and fifty to one thousand diameters, and with 

 a substage bearing an Abbe light -condensing apparatus (to which an iris diaphragm 

 should be attached for a convenient regulation of light). The student of bacteriologic 

 technique may be supposed to have become familiar, from his previous histologic 

 or biologic work, with the use of the microscope ordinarily, and for this reason 

 no discussion of the principles involved in its construction and manipulation need 

 be entered into here. It may be opportunely suggested, however, that in the examina- 

 tion of fresh, unstained bacteriologic specimens the diaphragm of the light apparatus 

 should be contracted as much as possible consistent with necessary illumination 

 in order to bring into sharp definition the outlines of the microorganisms, but that 

 for stained preparations the field should be flooded with light. The higher objectives 

 (one-tenth or one-twelfth inch focal distance) in common use are oil-immersion lenses, 

 a drop of an oil of refractive index nearly that of the glass of the lens and of the cover 

 and slide (thickened oil of cedar is usually used) being placed on the preparation, into 

 which the tip of the lens dips in focussing, in order to prevent loss of light. The 

 rays would otherwise be diverged on passing from the glass of the preparation into the 

 air between the cover and the lens and would be lost, and the accommodation for 

 light in these short focal distance lenses is so small that all the rays must be conserved 

 for efficiency. The amplification afforded, of course, does not depend only upon the 

 lens, but as well upon the eyepiece of the instrument ; but the most definite and clear 

 images are to be obtained with the objectives of short focal distance and oculars of 

 low power and high light capacity. With such an instrument one is enabled to make 

 all of the observations necessary for ordinary bacteriologic investigations; yet there 

 are organisms, as those met by Roux and Nocard in the infectious pleuropneumonia 

 of cattle, which require for their appreciation a much higher magnification than is 

 possible from the lenses suggested (these are barely recognizable with an amplification 

 of two thousand diameters) ; and the difficulty of recognition of the agencies in yellow 

 fever and other infectious diseases of man makes it quite reasonable to suspect that 

 the failure to detect them heretofore has largely depended on the minute size of the 

 organisms themselves. 



When using a low-power lens (one inch or one-half inch focal distance) it is not 

 essential that the material placed upon the slide be covered with the usual cover-slip, 

 all that is essential being that it be arranged jn an even moist film through which the 

 light may penetrate with uniformity. So, too, in examination of films which have been 



186 



