ILLUMINATION OF OBJECTS; ABBE CONDENSER 
23 
employ oblique illumination, and finally place diaphragms 
between object and mirror, noting well any changes in the appear¬ 
ances of the images seen. 
DEVICES FOR ILLUMINATION BY TRANSMITTED LIGHT. 
Condensers. — In order that sufficient light may enter a high- 
power objective to produce an image of such a degree of bright¬ 
ness as to be easily studied, it is essential that some device or 
apparatus shall collect, concentrate and send through the object 
light rays at an angle which will fill the aperture of the objective. 
The usual construction of this device is shown in diagram in 
Fig. 4 and is known as the Abbe condenser. Condensers of this 
construction with two lenses 
have usually a numerical aper- | 
ture, when employed to their 
full extent, of 1.20 and may be 
used with all ordinary dry ob¬ 
jectives and with oil immersion 
objectives. They are designed 
to be used with the plane mir¬ 
ror. In the case of objectives 
of more than 1.20 N.A., a three 
or more lens combination con¬ 
denser giving 1.40 N.A. should 
be chosen. Condensers used 
to their full aperture usually so 
flood the field with light, in the 
case of dry objectives, as to 
necessitate lowering them or 
closing their iris diaphragms 
or both until only just sufficient 
light rays are intercepted by I^i3,gram of Abbe Condenser; 
, 1 • • ,-11 • 1 1 Axial Light, 
the objective to fill its back 
lens and thus render the fine details of the illuminated object 
most distinct. 
In the diagram. Fig. 4, the passage of the light rays is roughly 
indicated for a position of the Abbe condenser when used with 
