illumination from the centre to the edges of the camera 
format, which of course increases the effective brightness 
range. 
2.7• Construction of Tables for use with the Maximum-Minimum 
Brightness Meter 
The method of constructing the tables referred to in 
paragraph 2.5* has been described by the writer in another 
paper 0). The principle is to dispose the maximum and 
minimum subject brightnesses at two negative density 
levels in such a way as to secure the highest possible 
resolution over the whole brightness range. Consider 
figure 1. Let M. and N. be two density levels which for 
a particular film confine resolving power to certain 
acceptable limits and let A. and B. be maximum and minimum 
subject brightnesses. The object is to spread the 
subject brightness range exactly between these limits, 
such that the development gamma is The measured 
subject brightnesses A and B are shown plotted on the 
exposure scale with respect to the light flux that would 
enter the camera when the exposure is l/300th second at 
P.5.6. This i3 called the "standard exposure" and is 
used as a basis from which to deduce particular exposure. 
B 
—I 2.8. Shown also in figure 1 is a selection of character 
istic curves for various emulsions developed to produce 
_ different gammas. The different positions of these 
curves on the log exposure axis is indicative of their 
various speeds. A curve must now be chosen, having a 
slope or gamma equal to Lid and the brightness range AB 
must be shifted exactly into position under this curve. 
The amount of this shift is equal to the difference between 
the "standard exposure", and the exposure to give the 
desired negative densities. Each 0.3 shift is equivalent 
to a change of exposure of one stop. The exposure 
required is determined in this way but with two further 
criterions 
1 • The change of exposure indicated by the shift 
must be possible for the camera being used 
without decreasing the shutter speed beyond the 
limits necessary to avoid significant image 
movement. 
2. Where there are various alternative possibilities, 
one chooses the emulsion having the finest grain. 
Tables constructed on this basis would enable a photographer 
to read off opposite the entry of his maximum/minimum meter 
readings, the best film to use, the exposure to give to it, 
and the development time to match that exposure. In 
practice it would be found unnecessary to consider more 
than two different classes of air film.