88 METHODS OF OBSERVING BACTERIA 



containing the optical apparatus, which can be moved up and down 

 on a supporting arm from an upright pillar of the stand. The raising 

 and lowering of this tube is accomplished by a rack and pinion, 

 worked by a large screw-head on either side. This arrangement is 

 called the coarse adjustment. In the outer tube is an inner tube 

 which can be drawn out by hand. This inner tube is graduated in 

 millimeters, and if the lower end of the outer tube is provided with 

 a so-called revolver or nosepiece the inner tube should be drawn 

 out to the mark 16 cm. or 160 mm. The microscope will then be 

 so adjusted that the distance from the upper lens of the eyepiece to 

 the objective is 170 mm. (170 millimeters = 17 centimeters). This 

 is the distance to which the higher power objectives are corrected 

 and at which they work best, giving the clearest picture of the object 

 under observation. 



There is, in addition to the coarse adjustment, the so-called fine 

 adjustment worked by a micrometer screw, placed either on top or 

 at the sides of the centre pillar of the instrument. This raises and 

 lowers the draw tube only very slightly so that it can be adjusted 

 to the one-hundredth part of a millimeter. 



Condenser. One of the most important parts of the instrument is 

 the substage or Abbe condenser, 1 or illuminating apparatus. This 

 is attached below the central opening of the microscope stage in 

 such a manner that it has considerable movement in a vertial plane, 

 allowing the upper lens of the condenser to be on a level with the 

 stage when desired. The importance of this vertical movement will 

 be seen presently. The substage condenser consists of three parts, 

 namely, a reflector, an iris diaphragm and the condenser lens proper. 



The reflector is a circular disk suspended so that it can revolve 

 around the median axis of the equatorial plane, and has two sur- 

 faces, a plane mirror and a concave mirror. The condenser proper, 

 which forms the upper part of the illuminating apparatus, consists 

 of a system of lenses. Between the reflector and the condenser lens 

 is an iris diaphragm. The word diaphragm means a partition wall, 

 and in the modern microscope this wall between the reflector and 

 the condenser opens and closes in a concentric manner like the iris 

 of the eye, hence the name. Those who have done photographic work 

 with the camera understand the workings of the iris diaphragm; it 

 can be adjusted so that it forms a large opening, admitting a power- 

 ful bundle or pencil of rays of light, or it can be closed through all the 

 intermediate stages to a pinhole opening admitting very little light. 

 Some of the modern microscopes possess a second iris diaphragm 

 placed above the substage condenser lens. This second diaphragm 

 is used in place of the lower one when the condenser lens has been 

 removed. When instruments are equipped with Abbe condensers 

 the plane mirror of the reflector should be used and not the concave 

 mirror. 



1 Called Abbe condenser because invented and perfected by Professor Abbe, of Jena. 



