Components of the Two-Lens Abbe Condenser.

In most Abbe condensers, the top lens can be unscrewed from the field lens, which can itself be unscrewed and removed from the mount with diaphragm.
The strongly curved top-lens is seen on the extreme left, then the less powerful field lens (which can act alone as a low power condenser of about twice the focal length).
The remaining portion consists of the substage stop carrier, the substage iris diaphragm, and the cylindrical sleeve which locates the condenser in the racking substage mount of the microscope. A small locating screw in the cylindrical fitting locates in a slot on the racking unit to ensure that the diaphragm lever and stop-carrier lever are always located in the same position.
The lenses can safely be cleaned by breathing lightly on the lens surface and wiping the surface firmly with a Kleenex tissue. Dusty condenser lenses (up to a point) have little effect upon image quality except for dry darkfield setups where dust on the toplens will catch the light and cause flare in the black background. Scratches on the toplens will have the same effect.

The field lens of this particular condenser has an aspherical lower surface which significantly improves the correction for spherical aberration of both the field lens and of the complete condenser. Low power objectives of up to 0.4 NA can be fully utilized using the field lens alone, and objectives of up to 0.65 NA can be fully illuminated using the complete condenser.
This has no effect however upon the poor state of chromatic correction characteristic of this condenser design.