These water immersion objectives are used especially in physiology, in conjunction with the upright microscope, for samples such as thick brain sections. On account of their thickness, such samples cannot be observed with inverted Axiovert microscopes and LD objectives, contrary to the examination of cell cultures in transparent vessels. The upright Axioskop 2 FS makes it possible to immerse the objective into a medium contained, e.g., in a Petri dish and examine the specimen from above. Focusing of the Axioskop 2 FS acts on the nosepiece instead of the stage (FS = Fixed Stage).These objectives are, of course, also used for transmitted-light microscopy. One example is the draining of electrical potential from cells. “Achroplan” water immersion objectives are specially adapted to requirements in electrophysiology (patch-clamp technique), in intravital microscopy, in microcirculation studies, in the examination of vital brain sections and other thick specimens. In these techniques, some fundamental conditions have to be satisfied: Superb optical performance and transmission, slender form, front part of special material and long working distance. All Water Achroplan objectives have fields flattened across a diameter of 20 mm. They allow all contrasting methods, including high-resolution differential interference contrast (DIC). Thanks to their high numerical apertures, they not only provide excellent images but also are ideal for fluorescence - not least because of the use of special glass types that are highly transmissive to the excitation light. Further, these objectives are suitable for the "INFRAPATCH" method at wavelengths up to 780 nm (special IR versions are available). The slender front part of the objective and the long working distance of these objectives provide sufficient space for the specimen to be well accessible to all microtools (patch-clamp electrodes, microinjection capillaries etc.). It is possible to implement big positioning angles to minimize self-capacitance in patch-clamp potential draining. Moreover, the slender front part barrel is made of special, insulating plastics of extremely low self-capacitance. The material is inert, i.e. it does not release metal ions to the surrounding medium; it is corrosion-proof, chemically resistant and of minimum thermal conductivity.