ILLUMINATION OF OBJECTS; DARK FIELD 
47 
test slide may now be replaced by the preparation to be studied. 
Little change, if any, should be required to give the most sat¬ 
isfactory results. If material of unknown structure or com¬ 
position is placed upon the stage without a prior examination 
of material of known behavior much time may be lost in attempt¬ 
ing to interpret anomalous appearances due to improper illu¬ 
mination. 
Owing to the exceedingly complicated diffraction patterns 
often obtained with dark-field illumination great difficulty may 
be experienced in arriving at a correct explanation of the phenom¬ 
ena observed, and it is only after study of materials of known 
structure that it is safe to proceed to examinations of somewhat 
similar material of unknown structure. 
/. Orthogonal Illumination is a term applied by Zeiss after 
Siedentopf and Zsigmondy to an arrangement of radiant, con¬ 
densing lenses and tiny slit such that the light rays enter the 
preparation at right angles to the optic axis of the microscope. 
The presence of particles is thus indicated by the light diffracted 
from them, the particles themselves remaining invisible and only 
the diffraction patterns, which may be relatively large, are seen 
in the field of view. This mode of illumination, applied to 
microscopic examinations, gives us instruments commonly called 
slit-ultramicroscopes. 
Orthogonal illumination is employed in the study of colloids 
and other particles in suspension in liquids and for the study of 
particles in transparent or translucent solids, such as glass, etc., 
and for the investigation of vapors and gases. 
g. Differential Color Illumination by the method of Rhein- 
berger ^ may be obtained by substituting for the dark-ground 
wheel stop of the Abbe condenser colored disks of transparent 
material, using a darker color for the central portion and sur¬ 
rounding this disk with an annular ring of a lighter and strongly 
contrasting color. The object will then appear strongly illu¬ 
minated, but colored upon a colored background. If, for 
example, the central disk is blue and the ring red, the objects 
will appear red upon a blue background. With care and a 
^ J. Roy. Micro. Soc., 1896, 373; Spitta, Microscopy, London, 1099; 175-178. 
