which gives the same working aperture. The following rule is a standard on this point: " Depth of focus and definition are opposed to rapidity. Whatever increases the rapidity of a lens reduces the power of definition, and conversely any gain in definition and depth, granting the lens to be well made in other respects, must be made at the expense of rapidity." Definition and depth are thus opposed to rapidity. As rapidity is increased with larger aperture, definition and depth are lost, till a point is reached at which the extent of such loss prevents the further increase of rapidity. Supposing a lens is being used which works atand it is desired to substitute a lens working at
and the exposure for a given subject is known with the former, the increase of exposure is found by squaring and com-paring these numbers: There-fore the exposure will be as 64 to 484, or 1 to 7*5. In this way the necessary increase in exposure for any size aperture may be found.
Or reducing these, and reckoning the exposure necessary with as unity, the exposures will be - I, 2-25, 4, 7*9, 16, 30*25, 64, 126*5, 256. (See Exposure, Diaphragms, and Lens.)
Rayometer. A kind of actinometer (generally a screen of aluminium of graduated thickness) used in testing sensitiveness to X rays. (See Radiography.)
Reaumur. See Thermometer.
Red Fog. See Fog.
Redevelopment. A process usually confined to the wet collodion. It is actually intensification of the image. It has been suggested, however, for dry plates and bromide papers, and is useful in the former case to intensify under-developed negatives, and in the latter to obtain warmer tones. The negative to be intensified is washed thoroughly free from hypo, and bleached in a solution of cupric or mercuric chloride, well washed, and then redeveloped with hydroquinone or ferrous oxalate. To make a solution of cupric chloride : -