MOUNTS, LENS Photographic lenses are usually mounted in brass, but in certain cases, especially with tele photo combinations, the lighter metals, alumin ium or magnalium, may be used. The iris dia phragm should be made of metal, especially in a lens for use in an enlarging lantern, as the heat is liable to distort, and even to destroy, iris leaves constructed of vulcanite or ebonite. Lenses should always be fitted with the standard threads of the Royal Photographic Society, and in the case of Continental makes it is convenient to have a fixed adapter made to the next larger standard thread. The lens hood may be con sidered a part of the mount ; unfortunately, it has now fallen into disuse, but in these days of large apertures and rapid plates it is more than ever necessary ; however, it is obtainable as a separate accessory, which may be made to serve for several lenses.
The following are the measurements and standards recommended by the Royal Photo graphic Society. The recommendations are :— ( ) That the equivalent focal length of a lens be engraved upon its mount.
(2) That the following series of screws for photographic lens flange fittings be adopted, it being understood that, in order to secure free interchangeability, every male screw should be made at least as small as these sizes and every female screw at least as large :— The form of thread is that known as Whit worth's angular thread, and is designed as follows :—Two parallel lines, at a distance apart equal to •96 of the screw pitch, are intersected by lines inclined to each other at fifty-five degrees, as shown in the figure at A. One
sixth of the vertical height of the triangular spaces so obtained is rounded off both at the top and bottom, leaving the form of the screw thread as at B. The depth of this thread is -64 of the screw pitch. This is the theoretical form of the Whitworth thread, but for the purpose of securing real interchangeability it is generally found necessary to use chasers or other threading tools which have additional prominence upon those points which come first into operation and are subject to most wear. For this purpose an addition may be made to the amount of one-tenth (a) of the theoretical depth of thread or to any less amount that may be sufficient.
(3) That every flange and adapter have a mark upon its front to indicate the position of the diaphragm slot or index of any lens when screwed home. The mark on any adapter should coincide with the mark upon any flange into which it is screwed. This mark should be placed at the point at which the thread becomes complete at the shoulder of the flange or adapter.
Of MA:WI The meto-hydroquinone developer (which see) " M" indicating metol, and " Q " hydroquinone (quinol).