RADICAL COr1POUND.—In organic chemistry a compound -adical is a group of elements which, in the various changes and lecompositions which a substance undergoes, remains unaffected, ind acts as if it were an element. Cyanogen, ethyl, etc.. are corn iound radicals.
RAPIDITY.—The rapidity of a lens is dependent upon the relation the focus bears to the working aperture. The rapidity of the photographic dry plate is usually known by its sensito meter number, but this is often very misleading, however. The rapidity is usually obtained by heating the bromide emulsion for some time or by the ammonia process.
RAPID LENS.—Any lens allowing a large quantity of light rays to pass through upon the sensitive plate. The rapidity of a lens is dependent upon the relation the focus bears to the size of working aperture. Depth of focus and definition are opposed to rapidity, and, therefore, any arrangement tending to increase the depth and definition can only be made at the expense of rapidity. It is generally understood that the rapidity of all lenses is equal with the same ratio
size diaphragm. Although this system holds good with single lenses, through which the same amount of light is always admitted through the same size aperture of diaphragm, yet with com binations of two lenses, a certain distance apart, as in rapid rectilinear, etc., this cannot be said to be quite correct. Here the ratio of the focus to the actual aperture no longer regulates the fa rapidity. The available aperture (A) is in this case given by the equation A = being the the focal length, a the actual aperture of the diaphragm, and d the distance between the center of the diaphragm and the nodal point of emission of the back combination.