90 METHODS OF OBSERVING BACTERIA 



Lenses. The objectives commonly used in work in histology, 

 pathology, and bacteriology generally have focal distances of two- 

 thirds inch, one-sixth inch, and one-twelfth inch (16 mm., 4 mm., 

 2 mm.), and they magnify, if used with the proper eyepieces, about 

 80, 400, and 800 times or linear diameters. 



Dry Lenses. The first two of these three lenses are used without 

 the interposition of any fluid between the cover-glass of the prep- 

 aration and the front lens of the objective, and are called dry lenses. 



Immersion Lenses. The third one, that which has the highest 

 magnification and the shortest focal distance, is used with a drop 

 of thickened cedar oil, called homogeneous immersion oil, interposed 

 between the cover-glass of the preparation and the front lens of the 

 objective. Lenses used in this manner are called homogeneous immer- 

 sion lenses. High-power lenses are best constructed as immersion 

 lenses for optical reasons. When a preparation is examined the light 

 is first reflected upward by the mirror of the substage illuminating 

 apparatus, it then passes the opening in the iris diaphragm, enters 

 the condenser proper, and is focussed by it upon the microscopic 

 object to be examined. From the latter the rays of light enter the 

 objective of the microscope. In doing so, when a dry lens is used, 

 the rays of light on leaving the cover-glass of the microscopic prep- 

 aration, pass through a small amount of air before reaching the 

 lower lens of the objective, and are refracted in such a manner that a 

 considerable portion of the light is lost. This loss of light and the 

 decrease in the angle of aperture under which the rays enter the 

 objective cause loss of clearness and detail in the picture. If, instead 

 of air between the cover-glass and the front lens of the objective 

 there is a drop of thickened cedar oil, which acts toward light in 

 the same manner as the cover-glass of the microscopic preparation, 

 or, in other words, possesses the same refractive index as ordinary 

 glass, there will be no loss of light, but a larger angle of aperture of 

 the entering rays and a much better image. Therefore, high-power 

 lenses are constructed as homogeneous oil-immersion lenses, and such 

 a lens with a numerical aperture angle of 1.4 is better than one 

 with a numerical aperture of 1.3. Homogeneous immersion lenses 

 have a very short focal distance (2 mm. = T T inch); hence, if a 

 thick cover-glass be used it will be impossible to bring the lens within 

 the proper focal distance of the objects (bacteria) to be looked at. 

 It is, therefore, necessary to use, in work with bacteria, the thinnest 

 cover-glasses, that is, No. 1. No. 2 may sometimes answer, but it 

 is not safe to use them, and the student should always see that he 

 uses for work with bacteria the thinnest kind. 



Focussing. In order to see clearly with the microscope, it is always 

 necessary to lower or raise the tube in such a manner that the ob- 

 ject has the proper focal distance from the front lens of the objective. 

 The proper adjustment must be judged by the eye. This manipu- 

 lation of the instrument to obtain a clear picture is called focussing, 



