ILLUMINATION OF OBJECTS; DARK FIELD 
45 
the objective which is not corrected for the thickness of the 
cover of the cell being used. 
A powerful source of light is essential. Direct sunlight by- 
means of a clockwork heliostat is ideal but seldom available. 
The next choice is an electric arc of 4 to 5 amperes or more, for 
ordinary dark-field examinations, and of 15 to 20 amperes for 
ultramicroscopic studies of colloids, etc. Useful types of radi¬ 
ants will be found described on page 163.^ 
The more powerful the radiant the smaller the particles which 
can be demonstrated. Siedentopf estimates that direct sunlight 
will reveal the presence of particles whose diameters are one- 
thirtieth of that of the smallest appreciable with the ordinary 
arc lamp. 
Since the light rays enter these reflecting condensers through 
an annular space, there being an opaque stop at the center, it 
is obvious that the spot of light reflected from the mirror of the 
microscope must have a diameter slightly greater than this 
space, otherwise the illuminator will not properly function; for 
this reason, before placing the illuminator in position for cen¬ 
tering, it is always essential to examine its lower surface and 
ascertain the diameter of the spot of light necessary to com¬ 
pletely fill the annular entrance space. The radiant and a suit¬ 
able condensing lens are then so placed as to yield parallel rays 
and produce a spot of light of the proper size and intensity at 
the center of the plane mirror of the microscope, the mirror being 
so inclined as to reflect the light rays into the dark-field illu¬ 
minator. Dark-field illuminators require that an immersion 
fluid be placed between them and the object slide. To obtain 
the best results homogenous immersion oil should be employed, 
water seldom yields good results. 
In applying the immersion fluid and laying the object slide 
in place great care must be taken to prevent the entrance of 
air bubbles or dust particles. 
Because the light rays are caused to emerge from the illumina¬ 
tor at such an angle (determined by the inclination of the reflect- 
1 The “ Chalet Lamp ” of the Bausch & Lomb Optical Co. is of especial con¬ 
venience and value with dark-field illuminators. 
