18 Microbes and You 



Age of Microbiology, commonly designated as the period between 

 1850 and 1900. 



As we increase the magnification of our lenses with a light-type 

 instrument, we must have a greater concentration of light if we are 

 to see our objects clearly. An important contribution in this di- 

 rection was the introduction of the immersion lens which provides 

 a homogeneous refraction system for the light as it passes from 



Fig. 4. Ferdinand Cohn (1828-1898). (By permission from In- 

 troduction to the Bacteria, by C. E. Clifton. Copyriglit, 1950. Mc- 

 Graw-Hill Book Company, Inc.) 



below up through the lenses. In other words, by placing a drop 

 of oil on top of the preparation on the slide to be examined, if the 

 oil has the same index of refraction as the lenses of the microscope, 

 the light coming from below will not be lost after hitting the 

 object, but will continue through the oil and will be reflected 

 through the objective lens of the instrument. Along with this 

 improvement, the perfection of the substage condenser by Ernst 

 Abbe (1840-1905) about 1870 made possible a more brilliant 

 illumination of the microscopic field. 



With the use of improved microscopes observers were better 



