ILLUMINATION OF OBJECTS; DARK FIELD 37 
denser ” and may be employed for demonstrating the presence 
of particles less than 0.2 ju in size. 
Dark-field illumination is employed in practice in the exami¬ 
nation of blood for the presence of parasitic organisms, in the 
study of bacteria, in the biological examination of water, in 
the study of foods, fibers, crystallization phenomena, tiny crys¬ 
tals, submicroscopic particles, colloids, etc. 
If the Abbe condenser is to be . employed for dark-field illu¬ 
mination, insert one of the dark ground stops in the ring attached 
to the bottom of the condenser mounting, open the iris dia¬ 
phragm to its full capacity, and screw up the condenser in its 
mounting until, when turned in place and the substage is racked 
up to its highest point, the upper lens will just touch a slide 
laid upon the stage. A drop of water is then placed between 
the condenser lens and the preparation to be examined. It 
is always essential to ascertain the thickness of object slides 
which yield the best results and keep this value for future refer¬ 
ence. Special dark-field illuminators are marked by the manu¬ 
facturers with the thickness of object slide for which they are 
designed. 
The use of the Abbe condenser with dark-field stop as a sub¬ 
stitute for special dark-field illuminators is not to be recom¬ 
mended since the obliquity of the rays is seldom sufficient to 
prevent some light from entering the objective. The results 
usually obtained are poor and unsatisfactory. 
Dark-field Illuminators are condensers of such construction 
that very oblique Hght rays are caused to converge, usually by 
reflection. 
The rays either pass through the preparation at an angle 
with the perpendicular so great that they fail to enter the objec¬ 
tive (providing it is of low numerical aperture) or they strike 
the cover glass at such an angle as to be reflected downward 
and therefore fail to enter the objective. When no object lies 
in the field and no fine particles occur in the mounting medium 
between slide and cover glass, the field of the microscope is 
uniformly dark. In order that there may be no change in 
direction (through refraction) of the rays emerging from the 
