This is a darkfield condenser designed to be used with immersion oil. The maximum numerical aperture of 1.36 will accommodate higher magnification objective lenses with numerical apertures up to 1.25. However, it is recommended not to exceed NA 1.0 for true darkfield, rather than using darkfield objective lenses, such as those with iris control. Also, magnifications of 10X and below may experience vignetting, in which case a dry condenser is preferable.
The darkfield condenser uses light diffraction principles to suppress field illumination, while illuminating the edges of transparent or translucent objects. This is particularly useful for quantitative analysis of low contrast samples.