Double-Gauss lens objective lens by adding a meniscus lens to its single convex and concave lens design. Alvan Clark and Bausch & Lomb further refined the design in 1888 by taking two of these lenses and placing them back to back making a "double Gauss" with indifferent photographic results. Current double Gauss lenses can be traced back to an 1895 improved design, when Paul Rudolph of Carl Zeiss Jena thickened the interior negative menisci and converted to them to cemented doublets of two elements of equal refraction but differing dispersion in the "Zeiss Planar" of 1896 to correct for chromatic aberration. It
Compound refractive lens the order of meters, by using many lenses in series. To minimize the absorption the lens must be made from a material such as aluminium, beryllium, or lithium. CRLs were first demonstrated in the mid-1990s by a group of scientists at the ESRF. They drilled holes in an aluminium block, and achieved focusing in two dimensions. For X-rays a concave lens focuses the X-rays because the index of refraction is slightly below unity. In a CRL of this type the walls between the cylindrical holes act as concave lenses for X-rays traveling perpendicular to the axis of the drilled cylinders.