TINTING MEDIUMS OTHER THAN THE STANDARD It has been seen that in order to measure the actinicity of the sky with the standard film emulsion in the standard meter the time re quired to get the standard tint is inconveniently short. It is also true that the great sensitiveness and the consequent short time required to tint the standard film in strong exterior light makes the examination and detection of the least visible tint difficult. It is evident therefore that under such conditions an advantage would be gained if some slower tinting medium were used in the meter. In order to obtain the standard first appearance time from the use of any other slower or faster medium it is only necessary to know the relative tinting speeds of the two mediums and this may be known by simply comparing the speeds of those mediums with each other. For example under certain con ditions the standard emulsion reveals a time of 1 second while on placing a strip of gelatine printing out paper (p.o.p.) in the meter 8 seconds are required to get the tint. These are the papers of the class represented by "citrate," "disco," "solio," "sunrae," etc., and they will all be found of practically the same tinting speed. Considering then the standard emul
sion as having a tinting factor of one, p.o.p. will have a factor 8 and on any occasion by measur ing the light with the slower medium, the time found, divided by 8, will give the standard time sought. (See also pp. 41, 156.) Thus in the case of the sky measurement previously made in which it was necessary to halve the meter opening, on using a strip of the slower tinting medium in the meter the time would have been found to be 1 second and this time divided by 8, the factor of the slow paper, would give the standard time of 1/8 seconds, the same as when measured with the standard emulsion.
It results also that with this slower medium a first appearance time of 512 seconds or 8 minutes indicates a one actino intensity just as 64 seconds or 1 minute indicates that intensity with the standard tinting medium and conse quently that a one second time with the slower medium indicates a 512 actino intensity in the surface measured. This latter is proved by the sky measurement just made.