(476) 
etc.) we are obliged to provide ourselves with the most stable emulsion-carrier. 
The choice between film and plates is equally important for official and private 
surveying enterprisers as for the instrument-manufacturors. We in Switzerland 
have chosen plates because the irregular image deformation on film is three 
or four times greater. Our theoretical and practical experiments show that in 
the actual state of the optical and mechanical design of instruments, full use 
of their precision will only be possible by using plates or by a considerable 
improvement of the stability of film. 
Mr. Poivilliers: To my view the publication of Prof. Kaspers is an out- 
standing one. I completely agree with the contents especially because they 
confirm with our experience. This is also the reason why in France, already 
years ago, glass-plates were used to a great extent. 
Mr. Corten: We have heard reports about excellent results with measure- 
ments on film from Mr. Brucklacher. There are several publications that 
confirm these results, but there are others which confirm the results we have 
had in Holland and that are of opposite character. If we ask for the source of 
this difference of opinion we have to look to the physical side of the problem 
and not to the photogrammetrical one. 
Essential is what is physically happening to the film during exposure, 
development and so on. We can distinguish between two different methods of 
approach to the problem of film-shrinkage. The physical process to which the 
film is subjected between manufacturing and restitution is far too complicated 
to be exactly investigated. The photogrammetrical point of view is only based 
on the final result and if the influences of some of the foregoing stages add up 
unfavourably, the photogrammetrist feels very unhappy. We can't expect 
from the film the same results as from glass-plates. For a great part of the 
applications of aerial photographs the film as it is manufactured now will give 
sufficient accurate results. If we put more rigid requirements it will be neces- 
sary to provide a complete air-conditioning in all phases where film is used. 
There will still remain small irregular shrinkages so that in those cases where 
the utmost precision is necessary only glass-plates will give the solution. 
Mr. Reading: There is a new flexible emulsion-base, and as I understand 
a factory is being built to produce this material, and it may be expected that 
this will do away with our film-difficulties. I tried to get a communication about 
this material for the congress but the manufacturer preferred to wait until he 
had got some in production. This is something we should consider in future. 
Prof. Schermerborn: According to our experience in many cases film bases 
are blamed for irregularities which are only due to lack of flatness of the film 
at the moment of the exposure. Therefore no film base will solve the problems 
as long as we can't be sure about this flatness. 
Following this discussion a communication is given by Mr. Nistri about: 
“Device for recording the nadir position directly on the photograph at the 
exposure.” 
Text see page (381)-109-1. 
Discussion. 
Mr. Dubuisson: The solution that Mr. Nistri gave for the registration of 
the nadir point seems to me a very excellent one. We have in France some