OPTICAL THEORY OF LIGHT MICROSCOPE 



For more exacting work in microscopy or 

 for photomicrography, an achromatic 1.40 

 N.A. condenser is often used. This is ciuite 

 well corrected for spherical and chromatic 

 aberration and permits more accurate con- 

 trol of the illuminated field and aperture, as 

 well as giving more color-free illumination 

 than is possible with the Abbe condenser. 

 Its construction is shown at the right in 

 Fig. 5. 



Darkfield Illumination. This is another 

 common system of illuminating microprepa- 

 rations. In this system the illuminating cone 

 is again concentrated on the specimen, but 

 in this case is hollow, having a dark central 

 core, as shown in Fig. 6. The objective lies 

 in this dark central core, and "sees" only 

 objects which scatter light onto it. The clear 

 background appears black, and the specimen 

 shines bright against this dark background. 

 This very striking form of illumination is 

 useful on very small transparent living ob- 

 jects such as spirochetes, bacilli, etc. 



The most common form of darkfield illu- 

 minator is the Paraboloid Condenser, shown 

 at the left in Fig. 6. A somewhat more com- 

 plex, and slightly superior design is the Car- 

 dioid Condenser, shown at the right in Fig. 6. 

 The Paraboloid delivers a hollow cone lying 

 between the approximate N.A. values 1.15 



and 1.40. The cardioid delivers a somewhat 

 better concentrated hollow cone lying be- 

 tween the approximate N.A. values 1.20 and 

 1.40. Since in both cases the N.A.'s exceed 

 unity, it is obvious that the condensers must 

 be oil-contacted to the glass object slide. 

 Since the objective N.A. must not be great 

 enough to accept any of the direct cone of 

 light, it is necessary to either use objectives 

 lower in N.A. than about 1.0, or else reduce 

 the N.A. to about 1.0 by a small baffle, 

 called a "funnel stop", which fits into the 

 back of an objective. 



The effectiveness of a darkfield sj^stem is 

 dependent on the use of an intense, nondif- 

 fused, light beam from the lamp condenser. 

 Carbon arcs, ribbon filaments, or compact 

 coil filament lamps are normally employed as 

 the light sources. Extreme care must also be 

 taken to get the object slide and cover glass 

 clean, since any foreign objects will scatter 

 light and destroy the contrast. 



To carry the darkfield principle to its ex- 

 treme, a very intense beam of light may be 

 directed onto the object at approximately 

 right angles to the optical axis. Such an ar- 

 rangement is called an ultramicroscope, the 

 name being derived from the fact that it is 

 possible with this instrument to see particles 

 which are well below the resolving power of 



I OBJECTIVE 



CARDIOID 

 CONDENSER 



PARABOLOID 

 CONDENSER 



Fig. 6. Two forms of darkfield condensers. Note that, in each case, the direct light from the con- 

 denser is not picked up by the objective. The scattered light from the object, indicated by the dashed 

 rays, produces a bright image seen against a dark background. 



451 



