90 The Microscope (chap. 9) 



moderate amount of light with parallel rays but no change in form; the 

 other concave to converge the light rays to fomi a cone, concentrating a 

 large amotmt of light. The latter replaces a condenser and can be used 

 for lower magnifications, btit should not be used when a condenser is 

 present and not for high magnification, when the plane mirror should 

 be used. 



Immediately below the microscope stage is the substage condenser, 

 whose lenses serve to converge the parallel beam of light from the 

 mirror so a cone of light passes through the aperture of the stage. This 

 cone of light is focused on the specimen tnider examination and then 

 is extended to fill the back lens of the objective. AV^ithoiu a condenser 

 the back lens of high-power objectives can not be filled with the proper 

 amount of light. By opening or closing the iris diaphragm moinited 

 below the condenser, the diameter of the light entering the condenser 

 can be controlled. A two-lens Abbe condenser is found on most 

 laboratory and student microscopes. For critical research microscopy, 

 a corrected condenser with a centering mount shotild be used. It should 

 be carefully centered to the objective and immersion oil shotild be 

 used between it and the slide for finest detail of the specimen. 



The image seen in the compotnid microscope is inverted, it is upside 

 down and turned right side to left. The movement of the slide will be 

 reversed. Most images depend for clarity on color in the specimen or 

 color added to it and/or differences in refractive indices in different 

 parts of the specimen and the medium in which it is motnited. 



There are innumerable types and sources of light used in laboratories. 

 If a table lamp is used, such as is fotmd in many sttident laboratories, 

 use a daylight btilb in it. Good, simple types of microscope lamps are 

 equipped with blue grotmd-glass filters. Place the lamp 10 to 15 inches 

 in front of the mirror and with all of the light directed below the stage 

 of the microscope. The stirface of the bulb or of the filter is focused on 

 the specimen by way of the stibstage condenser. If the lamp is the small 

 substage type of lamp, it may be used either in front of the mirror or, 

 with the mirror removed, is placed imder the condenser. A more critical 

 illumination (Kohler method) can be obtained with better kinds of 

 lamps provided with condensing lens and a coil or ribbon filament. 

 Controlled illumination is lighting by a cone of rays whose proportions 

 are regulated by a stop at the illuminator and by the iris diaphragm of 

 the condenser. The aperture of the oljjective is completely used and 

 none of the refracted light should fall on the lens mounting or dra^v 

 tube. 



