50 ELEMENTARY CHEMICAL MICROSCOPY 



trace of grease upon the glass surface will lead to the formation 

 of air bubbles, or will prevent optical contact if water is the im- 

 mersion fluid. 



The preparation to be studied must be thin and must be 

 covered with exceptionally clean and very thin cover glasses. 

 Covering the preparation with a cover glass is essential. 



In order to expedite the adjustment it is well to have at hand 

 a permanent slide of some material which yields good results 

 with dark-ground illumination, as, for example, diatomaceous 

 earth. With such a preparation on the stage the radiant, mi- 

 croscope mirror and the condenser are all so mutually arranged 

 as to yield the best illumination of the diatoms; the final adjust- 

 ment is then made by raising or lowering the condenser. The 

 test slide may now be replaced by the preparation to be studied. 

 Little change, if any, should be required to give the most sat- 

 isfactory results. If material of unknown structure or com- 

 position is placed upon the stage without a prior examination 

 of material of known behavior much time may be lost in 

 attempting to interpret anomalous appearances due to improper 

 illumination. 



Owing to the exceedingly complicated diffraction patterns often 

 obtained with dark-ground illumination great difficulty may be 

 experienced in arriving at a correct explanation of the phenom- 

 ena observed, and it is only after study of materials of known 

 structure that it is safe to proceed to examinations of somewhat 

 similar material of unknown structure. 



/. Orthogonal Illumination is a term applied by Zeiss after 

 Siedentopf and Zsigmondy to an arrangement of radiant, con- 

 densing lenses and tiny slit such that the light rays enter the 

 preparation at right angles to the optic axis of the microscope. 

 The presence of particles is thus indicated by the light diffracted 

 from them, the particles themselves remaining invisible and only 

 the diffraction patterns, which may be relatively large, are seen 

 in the field of view. This mode of illumination, as well as that 

 by exceptionally oblique rays, given above under e, applied to 

 microscopic examinations gives us instruments commonly called 

 ultramicroscopes . 



