THE STANDARD TINTING MEDIUM Many speeds of dry plates and films are made, i.e., many degrees of latent sensitiveness, but the author has found that the rate with which these emulsions visibly discolor on exposure to light is practically the same in all and for this reason all ordinary dry plate and film emulsions may be considered as standards for measuring actinism until an international standard shall be adopted. On account of the greater ease in working, the tinting strips will be cut for the most part from the small sizes of roll film, which fact will practi cally limit the standard to some half dozen different factories in the United States and in Europe. Avoid for this purpose films having any orthochromatic dye as they are thus rendered slower as to their tinting speed. A smooth bromide paper of sufficient tinting speed would be the ideal medium to use and some suitable paper might be taken as a standard. Although a standard of this character is not entirely satis factory to the author no inaccuracy can result in practice since each worker, should he follow out this method fully, will ascertain the speed of the plates or films which he uses, on the basis of the tinting medium which he may find con venient. Thus the method is found to present a very desirable elasticity in practice.
It is hoped that by the united efforts of the many photographic societies both here and abroad some specific tinting medium may be generally adopted as a standard. The flexi bility of this method is such that no difficulty whatever would be experienced in adopting a standard at any time.
Surfaces, which word will hereafter be used through lack of a more general one to express all light sources, as flames, surfaces and masses, such as the sky, clouds, etc., may be compared as to their actinicity by allowing the light from them to do a certain fixed amount of work through the standard opening of the actinom eter having a convergence of 4M cone units and the form of f / 1. As has already been ex plained, this fixed work shall be to discolor the standard tinting medium to the standard or least visible tint.
As a practical example, the walls of the room in which the writer is now working are in mod erate light and on adjusting a strip of standard film in the actinometer which has been de scribed, and exposing the meter for 64 seconds to a uniformly lighted part of the wall through the f / 1 opening, it is found that a decidedly deep tint was secured. On moving the strip
along to a fresh place and exposing for 8 seconds no tint was visible but 8 seconds more given to the same spot, making 16 seconds in all, pro duced the least visible tint. Now since an in tensity which will produce this tint in 64 seconds has been selected as the unit, this wall must be 4 units in intensity since it created the tint in 16 seconds or in one quarter of the time re quired for the unit intensity to do it. The rule for finding the actinicity of any surface which is possible to measure with the standard meter is therefore as follows: Rule: Divide 64, the number of seconds of first appearance time indicating one actino of inten sity, by the first appearance time of the surface as measured with the meter and the standard tinting medium; the quotient will be the inten sity of the surface measured in actinos.
As another practical example the intensity of the sky may now be found in actinos. Being much more actinic than the wall previously measured it will of course produce the tint in much less time, so it is first tested with a very short exposure, say of one second. Should the least visible tint be obtained by this exposure then the intensity of the sky would be found to be 64 actinos. But on examining the tint pro duced it is seen to be excessively dark and one second is therefore known to be much greater than the first appearance time. It is necessary then to give a shorter exposure than one second. On giving a quarter second exposure the tint is still seen to be rather dark, in fact it seems to resemble the second or third visible tint in the disappearing scale already described (p. 41). Now since one cannot make the next shorter or one eighth second exposure with the un aided hand it is found expedient to reduce the size of the opening by covering half of it with a straight edged black paper thus reducing its convergence from 4M to 2M cone units. When now another exposure of a quarter second is given, there results the least visible tint. This exposure is of course equivalent to one of 1/8 second with the full opening of 4M units. Ac cording to the rule, 64 divided by 1/8 gives the actinicity of the sky as 512 actinos.