CHAPTER XI 



OBLIQUE AND DARK GROUND ILLUMINATION, 

 CINEMATOMICROGRAPHY POLARISED LIGHT 



Oblique Illumination, The methods of illumination 

 hitherto described all depend on the employment of an 

 axial cone of light, and great stress has been laid on the 

 correct centration of the condenser. The wide-angle 

 cone of light projected by a substage condenser includes 

 very oblique rays, but they are directed on to the object 

 from all sides equally. What is usually understood as 

 " oblique illumination " is obtained by stopping out the 

 greater part of the cone of light, and allowing a pencil 

 from a portion only of the peripheral zones of the con- 

 denser to impinge on the object. The perfection of the 

 image given by a microscope depends on the numerical 

 aperture of both the objective and condenser, and 

 consequently it can hardly be expected that by obscuring 

 a great part of the aperture of the condenser an accurate 

 representation of minute structure is obtained, and, in 

 fact, the results must alwa}^ be regarded with great 

 caution, as false images, which may be easily taken for 

 a true representation of the structure under examination, 

 are very liable to be produced. Oblique illumination is, 

 however, sometimes employed in the resolution and study 

 of very minute periodic structures such as occur in 

 certain diatoms, though whether the appearance shown 

 represents the true structure of the diatom or is only a 

 diffraction effect must be decided by other methods of 

 research, one of which is afforded by ultra-violet light. 



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