80 ELEMENTARY CHEMICAL MICROSCOPY 



posed between radiant and illuminator or placed above the eye- 

 piece. 



Nernst, or other small filament lamps often fail to yield a 

 sufficiently even illumination; under such conditions a piece 

 of ground glass interposed between lamp and illuminator will 

 usually greatly improve the field of view, but will of course 

 reduce the brightness of image. 



To obtain satisfactory results in the study of opaque objects 

 with vertical illuminators it is important that the objectives 

 employed be constructed with compact mounts and that the 

 lenses be corrected for use with uncovered objects. Standard 

 microscope objectives are always corrected for some definite 

 cover-glass thickness. Moderate or high-power objectives of 

 this sort, therefore, cannot be employed for the study of 

 uncovered preparations. 



Most objective manufacturers supply special objectives for 

 use with vertical illuminators. Such objectives have very short 

 mounts and have the rear lens combination flush with the upper 

 edge of the mount (see Figs. 34 and 43). This is done to prevent 

 internal reflections and yields better fields and clearer and 

 brighter images. It is a safe rule to follow, if the best results are 

 wanted, to select an outfit in which the distance between the 

 reflecting surface of the illuminator and the rear lens combi- 

 nation of the objective is as small as possible. 



The diagrams, Figs. 34 and 37, have been drawn with a view 

 of showing this in an exaggerated way. In Fig. 34 a short com- 

 pact mount is shown, the rear lens combination is almost in 

 contact with the reflecting prism P, while in Fig. 37 an ordinary 

 objective is shown and the distance between reflecting disk F and 

 the rear lens is so excessive as will doubtless lead to interfering 

 reflections of an aggravated sort. With the construction shown 

 in Fig. 37, an objective with compact mount would be essential. 



The interior walls of vertical illuminators must never be 

 allowed to become bright but must be kept coated at all times 

 with a dull black finish. 



Since the diameter of the rear lens combination is different in 

 different objectives, especially when manufactured by different 



