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 
