The Nachet Substage Prism Oblique Illuminator was discussed in detail by Shadboldt and also the Nachet Company in: volume III of the Trans of the Microscopical Soc of Lond in 1850 on pages 74 through 82. Its chief advantage was that it could be rotated in the substage ring so that oblique light could be directed from any direction. Its disadvantages included its lack of achromatism, fixed angle of illumination, and the reduction in the diameter of the light beam entering it. This device was sold by Grunow as early as 1857, and for many years by Beck. The light path is illustrated in the image to the left, taken from Grunow's 1857 Catalog.