120 CONVERGENT VERSUS VERTICAL PHOTOGRAPHY, DISCUSSION 
invite our American colleagues to provide the 
audience with some hard facts and figures. That 
is a real contribution to the evaluation of the 
method. That is one question. 
Furthermore, you will find mentioned two 
different experiments. One is the famous Monti 
di Revöira which has been evaluated from the 
point of view of precision by the Swedish 
Society. Mr Möller was kind enough to give me 
an advance report about this. The mathematical 
statistician who works on this said that it is very 
difficult to come to any conclusion about it. 
Whether it is so dangerous, I doubt. 
I mentioned a few figures which I derived 
on the way I am using for this and found there 
that there is a great difference between the 
results of the convergent camera and of the 
vertical plate and film. We came there to a mean 
square error reduced to microns in the plane of 
the negative for signalised points to 17 microns 
for the plate, 27 microns for the film camera 
vertical which is also a high figure, and 34 
microns for the convergent photography. 
It may be that somebody who is viewing this 
from the point of view of statistics would say 
that you cannot come to any conclusions from 
this. For the time being I am inclined to say 
that all these figures are worth very little. I am 
afraid that there are other sources of errors in 
the picture in this case which in particular make 
these figures, I would say in my own terminol- 
ogy, useless, but that is rather a strong expres- 
sion and therefore it is better to say of very 
little use. 
In recent months we have experienced very 
bad things regarding films. I regret very much 
that I must admit this. We measured in Delft the 
fiducial marks of the Monti di Revóira pair be- 
cause also the Dutch results were unsatisfactory. 
A study of this finished in measuring the fiducial 
marks, measuring the distance, transforming 
on two points and then looking to see what is the 
difference in the position of the other two. I 
hardly have the courage to say what was the 
difference, but it went up to 60 microns. We 
cannot talk any more about 10 microns or some- 
thing like this, this makes no sense whatsoever. 
Another case of film still worse was where 
we recently had three rolls of films taken in a 
reseau camera. We made prints of two succes- 
sive photographs of the same roll, put them in 
the stereocomparator and measured the differ. 
ence in size of grid squares. The observed grid 
squares are with base O the same. The differ- 
ences went up 40 microns at the edges of the 
film. This are two pieces of film from the same 
roll at a distance of 25 centimetres. I will not 
say that the film is always bad, we also have had 
very good examples. You find these mentioned 
in my paper as experiments from Frankfurt, 
given to me by Dr Fôrstner. He has had better 
luck than we, and than the participants of the 
Monti di Revôira test. He finds very nice figures, 
7 microns for the convergent photography in the 
plane of the negative and for the vertical also 
7 microns. For the heights, derived from the 
verticals he found 13, and from the convergent 
as can be expected, a smaller value, 9 microns. 
There is nothing wrong. 
These combinations of figures make it very 
difficult to determine, at least on the basis of the 
solid material I had in hand at that time, which 
was the best. ; 
At the end a few additional remarks. I have 
always expected that one roll of film for both 
images, as it was in the pre-war Zeiss Twin 
camera, that was a better method than having 
two separate rolls as we have now. But after the 
bad experience with these three separate reseau 
films we are not sure about this. A point of 
doubt, however, is that these three rolls are not 
processed by a professional organisation, but by 
RAF people more or less as a training subject. 
But I am still even more than before in doubt 
as to what a film really means. There are several 
excellent publications about the high quality of 
film, but as I have said many times, I would like 
to look into the waste paper baskets of all those 
people who have made experiments to see 
whether they never discovered a film as bad as 
ours was in Delft. But we have a kind of 
privilege in always discovering bad phenomena. 
For the time being that is all I have to say. 
Now I would like to ask the audience whether 
somebody likes to raise questions. 
Discussion 
Prof B. HALLERT: I would like to ask some- 
body on the panel whether they have tested the 
basic geometric qualities of convergent pho- 
tographs. In other words, taking into account 
the different influence upon the geometry of, 
on the one hand, radial distortion from the lens 
and on the other hand curvature, reflection and 
perhaps other disturbances. This is a question, in 
other words, as to whether the basic regular or 
systematic errors have been investigated, and 
further whether the residual errors have been 
investigated. 
  
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