ELEMENTARY CHEMICAL MICROSCOPY 



Sections of typical illuminators are shown in Fig. 15, A, B, 

 C, D. It will be seen that although the construction may be 

 different in different types, the rays emerge at approximately 

 similar angles. In illuminators of these types (B, C, D) the 

 curvatures of the reflecting surfaces are ground after mathe- 

 matically calculated curves which will bring the light rays ap- 

 proximately to a focus at a point upon the cover glass. In the 



FIG. 15. Types of Dark-ground Illuminators. 

 A. Nachet et Fils. B. Reichert. 



C. Bausch & Lomb. D. E. Leitz. 



diagrams for simplicity, cover glasses and preparations have 

 been omitted. The cheaper forms of dark-ground illuminators 

 fail to bring the rays to a true focus and instead of a point of 

 light upon the cover glass we obtain a disk, as shown in an exag- 

 gerated manner in Fig. 14. 



An exception to the above statement, relative to the construc- 

 tion of reflecting condensers, is found in the Beck 1 dark-ground 

 1 Made by R. & J. Beck, London. 



