ONE of the many revolutions which have been quietly proceeding in the last few years has been the introduction of the prism-binocular in place of the old form of opera- or field-glass. In 1851 an Italian, Ignatius Porro, devised a very ingenious and yet simple arrangement of prisms by which the simple astronomical telescope might yield an erect image. An instrument was constructed with these prisms by Boulanger, in 1859, and again in 1875 by Nachet, the firm so well known in connection with the binocular microscope. Neither of those makers succeeded in making it popular, however, probably partly because of the quality of the glass of which the prisms were made, and partly because the prisms were not well enough worked to give good images— the light is four times reflected, and it is obvious that if the reflecting faces are not all perfectly flat the definition will be seriously impaired. In 1893 Ernst Abbe designed an instrument, making use of the new glass obtained by Schott; the resulting “prism-binoculars “made under the modern conditions were an immediate success. The faces of the prisms are tested by Newton's bands of colour. These bands must be perfectly straight right up to the edge. The refracting surfaces are tested, as well as the reflecting, though perfection of the latter is the more important.