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additional conditions that can be used to detect the excessive unknown errors. 
The horizon photography appears to work particularly well in the Arctic 
where the clear weather is usually free from haze. In the tropics the definite 
horizon is often difficult to photograph. 
From our nine lens experience I am sure a seven lens camera with wide 
angle lenses could be built to photograph everything visible from nadir to 
horizon in all directions. The overlap in all directions of the series of photo- 
graphs from such a camera would provide ready means of detecting and elimi- 
nating any local cause of excessive errors. 
It is very fine to have such free discussions with frank differences expres- 
sed as we have this morning, for that way lies progress. I remember from many 
examples in my own experience that scientists and engineers, no matter how 
conscientious and careful, simply cannot detect erroneous assumptions in work 
in which they have an intense personal interest. Everyone will find some rea- 
son to reject data which invalidates his much desired results. I remember a 
battleship admiral who insisted that the firepower of his big guns on one 
battleship would require nine aircraft carriers to equal in attacking power, al- 
though in the same speech, not ten minutes before, he had said, *The essence of 
naval strategy is to bring all your fire power to bear on the enemy before he 
can bring up his and thus destroy him piecemeal". That speech was made in 
1938. 
All of us have these mental blocks which prevent us from seeing dis- 
advantages in our methods or gaps in our assumptions, when our assumptions 
seem to support much desired results. The great advantage of science and the 
scientific discussions in meetings like this is that we can help each other to 
eliminate these mental blocks by noting differences and framing additional 
experiments to determine which direction or idea is best or nearest the truth. 
I am sure that I, and all of us, welcome the removal of any of our mental 
blocks even though the process may be a bit painful at the time. The frank, 
free discussions such as we have just had, point the way to the experiments 
needed for eliminating any ideas that may be holding back our progress. 
Mr. Reading's speech was applauded by the audience; after which the pre- 
sident closed the second session. 
THIRD SESSION 
Thursday September 11th 1952, 9.00—11.00 hr. 
Mr. Hoffmann (Germany) reads a paper prepared by Dr. J. Sutor (Ger- 
many) about *Triangulation from air photographs by distance measurement 
in the photograph". 
*Development of photogrammetry in the last ten years is marked by the 
endeavour to keep the tilt of air photographs as small as possible. The method of 
radial triangulation gains importance from this point of view. This method is 
based on measurement of angles. Special apparatuses are necessary to get precise 
results, which has found its expression in construction of radial triangulators. 
No corresponding method is known, however, which is based on distance 
measurement in air photographs, although distances can be measured easier 
and more quickly than angles.