Rapid Rectilinear Rapidity of Lenses
on the plate by exposure to the radiations from the vacuum tube. Similarly the shadow of an impacted needle can be cast on the plate. To detect the presence of a substance opaque to the X rays (or maybe a certain rank or degree of the cathode rays) it is not essential to use a photographic plate, as many substances fluoresce or phosphoresce (see note on fluorescence and phosphorescence under heading Luminous Paint) ; and if the shadow of a substance opaque to the rays is received on such a surface, it can be recognised as a dark area on a gene-rally illuminated ground. An abstract of all that has been wrilten regarding Radiography would fill many pages of this Dictionary, but we may give the following particulars as to the nature of the outfit required for ordinary work. An induction coil, giving a spark of about 4 in., and five pint cells of Bunsen's battery to work it, will cost, if of the best quality, from £18 to £20, and a suitable vacuum tube will cost from £1 to £1 10s. A fluorescent screen, consisting of paper covered with tungstate of calcium, or platino-cyanide of barium (or potassium), may cost from £1 to £2 ; and sensitive sheets of negative paper can be obtained, each in a separate black envelope ; a very con-venient arrangement when it is wished to radiograph for surgical purposes. Several sheets may be exposed at one time, the sensitive material being remarkably transparent to the exciting radiations. Radiographic work has now become rather a branch of the electrician's work than of photographic practice. Consequently for further details we must refer our readers to special handbooks. The following may be mentioned : " Prac-tical Radiography," by A. W. Isenthal and H. Snowden Ward, third ed., 1901 : London, Dawbarn & Ward; " Technique et Applications des Rayon X," par G. H. Niewenglowski, Paris, 1898 : H. Desforges.
Rapid Rectilinear. See Lens.
Rapidity of Lenses. The rapidity of a lens depends upon the relation the working aperture bears to the focus. It is an almost universally misunderstood question. Because a lens-maker calls any one lens rapid, it is generally supposed that such a lens is the quickest and most suitable for in-stantaneous or general work, whereas what is termed a wide-angle may be equally as rapid with the same size diaphragm,