ILLUMINATION OF OBJECTS; DARK FIELD 



43 



optic axis of the microscope ; (5) accurate centering of the objec- 

 tive. 1 



An examination of the diagrams (Figs. 14 and 19) will show 

 that theoretically the oblique rays meet to form a tiny spot of 

 light just outside the apparatus in the line of its optic axis. It 

 is obvious that this spot should lie in the optic axis of the objec- 

 tive and the ocular. In order to facilitate centering, a tiny circle 

 is usually engraved upon the upper surface of the glass of the 

 illuminator; this circle is focused with a low power and is brought 

 to the center of the field of the microscope, by means of center- 

 ing screws c, c, Fig. 20, provided for this purpose. 



When working with the Bausch & Lomb " Dark-ground 

 Illuminator " shown in Fig. 20, care should be taken to start 

 observations with the 

 diaphragm d, opened to 

 its full aperture. If the 

 preparation fails to yield 

 a satisfactory dark field 

 the diaphragm should 

 be slowly closed until 

 the best results are ob- 

 tained; a darker field 



and brighter particles will probably result, but the resolution 

 will be somewhat poorer. If, however, the diaphragm be closed 

 too far, as, for example, as shown in the right half of Fig. 20, 

 no light can enter the annular opening in the paraboloid and 

 the apparatus will fail to function. 



If the microscope is provided with a revolving nose-piece 

 the objective used in centering should be removed and the high 

 power to be employed in the dark-field studies substituted in 

 the same opening in order that there shall be no change in the 

 relations of the optic axes. When employing ultracondensers 

 of the highest type it is better to remove the nose-piece and to 

 attach to the body tube a centering adapter into which the objec- 



1 The adjustment and method of use of Dark-field Illuminators is discussed in 

 great detail by Gage. Trans. Amer. Micro. Soc. 39 (1920) 95. Workers with 

 dark-field illuminators should not fail to consult this paper. 



Fig. 20. Paraboloid Dark-field Illuminator. 



