determining the value of camera constant. The curve close to the origin is 
very sensitive to small experimental errors. However, the general form which 
the curve must take can be computed and it can then be fitted to experimental 
points at a sufficient distance from the origin to give accurate information on 
the limiting slope as the curve approaches the origin. Hence the camera 
constant can be found. 
R. A. Schlunde, South Africa: Gentlemen, I am referring to papers read 
on Saturday by Dr. Gardner and Dr. Washer about the test charts of the 
National Bureau of Standards. It is a definite improvement that low contrast 
test charts are being used, but I wonder why the improvement is going in the 
direction of low contrast in grey tones instead of in colours. For aerial photo 
graphs we have more or less always the same range of colours, greens, browns, 
and blues. Wouldn’t it be fairer to test a lens in those tones with high or low 
contrast? I wonder if Dr. Gardner would like to comment on that. 
Dr. Gardner was not present. 
F. L. Corten, Netherlands: Mr. Chairman, I have a question in connection 
with Mr. Cruset’s experiments about permissible image shifts. We know that 
there are considerable differences of opinion between the Americans, British, 
and French about the permissible image shift during exposure. The image shift 
which can be tolerated is quite important because it affects maximum permis 
sible speed and hence the economical choice of an aircraft. In the Netherlands 
we apply as a tolerance 0.02 millimetre during exposures. Consequently we 
need rather a slow aircraft. The general consensus in the United States, I think, 
is approximately the same. On the other hand, as I remember the publications 
by British investigators and by Mr. Cruset, there is an opinion that the per 
missible image movement during exposure is, let us say, 0.04 millimetre. This 
means that with the same camera, the same light and the same film it is possible 
to fly airplanes twice as fast. This is the case, for example, in Canada, and in 
the French photography in Africa. Could some further comments be made on 
this important point? 
Mr. Cruset, France: This is a matter which I would like to discuss with 
you fully when more time is available. In our case, at the I.G.N. we work on 
rather small scales. Photographs are taken for the regular map for France: at 
a scale of about 1:30,000; and for overseas territories: in North Africa, at 
1:40,000; in other territories of the French Union at 1:50,000 to 1:60,000. 
Now at these scales we do not worry about image movement. At the scales 
which would be required in other cases such as cadastral survey a special study 
would be required. 
Dr. Hewlett: Perhaps I take unfair advantage of my position to ask Mr. 
Corten what experimental data there is to suggest that the original relationships 
between resolution and movement during exposure derived by English workers 
are not correct. From their data anyone can determine what loss of resolution 
will occur for a given amount of image motion and hence can choose the most 
suitable shutter speed for their project.