4 6 



ELEMENTARY CHEMICAL MICROSCOPY 



e.g., Fig. 21, which lie upon the stage of the microscope. More- 

 over, all ultracondensers can be employed as ordinary dark- 

 ground illuminators, the only drawback in routine work being 

 that they require more careful adjustment. 



s \ 



FIG. 21. Simple Dark-ground Illuminator for Use upon the Stage of the Microscope. 



The Adjustment of Dark-ground Illuminators for use requires 

 close attention, chiefly, to four conditions: (i) a selection of a 

 sufficiently powerful radiant and the projection of a spot of 

 light large enough to completely fill the lower opening of the 

 illuminator; (2) the employment of objectives having a numer- 

 ical aperture never greater than i.o; (3) the use of object slides 

 of the thickness for which the illuminator has been designed; 

 (4) accurate centering of the illuminator with respect to the 

 optic axis of the microscope. 



An examination of the diagrams (Figs. 15 and 20) 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, either by 

 means of centering screws c, c, Fig. 22, provided for this pur- 

 pose, or is moved by the fingers when a stage illuminator, Fig. 

 21, is placed upon the stage. If the microscope is provided with 



