A condenser (right) and its respective diaphragm (left)
A condenser between the stage and mirror of a vintage microscope
Condensers are located above the light source and under the sample in an upright microscope, and above the stage and below the light source in an inverted microscope. They act to gather light from the microscope's light source and concentrate it into a cone of light that illuminates the specimen. The aperture and angle of the light cone must be adjusted (via the size of the diaphragm) for each different objective lens with different numerical apertures.
Condensers typically consist of a variable-aperture diaphragm and one or more lenses. Light from the illumination source of the microscope passes through the diaphragm and is focused by the lens(es) onto the specimen. After passing through the specimen the light diverges into an inverted cone to fill the front lens of the objective.
The first simple condensers were introduced on pre-achromatic microscopes in the 17th century. Robert Hookeused a combination of a salt water filled globe and a plano-convex lens, and shows in the 'Micrographia' that he understands the reasons for its efficiency. Makers in the 18th century such as Benjamin Martin, Adams and Jones understood the advantage of condensing the area of the light source to that of the area of the object on the stage. This was a simple plano-convex or bi-convex lens, or sometimes a combination of lenses. With the development of the modern achromatic objective in 1829, by Joseph Jackson Lister, the need for better condensers became increasingly apparent. By 1837, the use of the achromatic condenser was introduced in France, by Felix Dujardin, and Chevalier. English makers early took up this improvement, due to the obsession with resolving test objects such as diatoms and Nobert ruled gratings. By the late 1840s, English makers such as Ross, Powell and Smith; all could supply highly corrected condensers on their best stands, with proper centring and focus. It is erroneously stated that these developments were purely empirical - no-one can design a good achromatic, spherically corrected condenser relying only on empirics. On the Continent, in Germany, the corrected condenser was not considered either useful or essential, mainly due to a misunderstanding of the basic optical principles involved. Thus the leading German company, Carl Zeiss in Jena, offered nothing more than a very poor chromatic condenser into the late 1870s. French makers, such as Nachet, provided excellent achromatic condensers on their stands When the leading German bacteriologist, Robert Koch, complained to Ernst Abbe, that he was forced to buy a Seibert achromatic condenser for his Zeiss microscope, in order to make satisfactory photographs of bacteria, Abbe produced a very good achromatic design in 1878.
There are three types of condenser:
1. The chromatic condenser, such as the Abbe where no attempt is made to correct for spherical or chromatic aberration. It contains two lenses that produce an image of the light source that is surrounded by a blue and red color at its edges.
2. The aplanatic condenser is corrected for spherical aberration.
3. The compound achromatic condenser is corrected for both spherical and chromatic aberrations.