Mr. B. J. Attwell: In his experiments over the test chart of the island of 
Oland, has Prof. Hallert used both plates and film on the same camera. If he 
has used only film, how has he isolated the film distortion? We have found in 
our work on film distortion that we can determine systematic and random values 
for distortion of about the same magnitude as those given by Prof. Hallert for 
his ultimate distortions. 
Prof. B. Hallert: We have not yet tested one camera with different plates and 
films but that is of course very important. My suggestion here for this test is 
that it should be performed with film and plate. Of course, the film distortion 
is also included in the formulas. I think it might be of interest also to take a 
series of pictures on completely different films, to take the pictures in the same 
day and to take them on different days in order to get very different infor 
mation as to what the treatment of the film will mean, if other factors are con 
sequent. This is in the beginning. 
Mr. B. J. Attwell demande si le Prof. Hallert a utilisé à la fois des chambres à plaques et à 
film. S’il s’est servi de chambre à film, comment a-t-il tenu compte de la distorsion du film? Dans 
une étude de la déformation du film, Mr. Attwell a trouvé des valeurs systématiques et acciden 
telles dues à la distorsion du film qui sont de l’ordre de grandeur des valeurs finales indiquées par 
le Prof. Hallert. Le Prof. Hallert n’a pas fait d’essai avec une même chambre approvisionnée, soit 
de film, soit de plaques. Les formules tiennent naturellement compte de la déformation des films. 
Ces essais n’en sont qu’au commencement. 
M. J. Cruset: La comparaison des plaques et des films sera certainement re 
prise à la quatrième session, lundi matin. Je ne fais qu’anticiper un peu en 
soulignant que la distorsion du film n’est pas seule en cause et que la variation 
de sa forme géométrique est une question encore plus importante. Il s’agit de la 
différence entre la forme géométrique qu’avait le film au moment de la prise 
de vue et de la forme géométrique qu’il a quand il est exploité sur l’appareil 
de restitution. On connait mal, notamment, la forme exacte du film au moment 
où il est exposé dans la chambre de prise de vues. 
Mr. Cruset informs the audience that the comparison between plates and film will be certainly 
taken up again at the fourth session. Anticipating on that farther discussion, he wishes to stress 
that film distortion is not the only point, but that differences of the film’s geometrical form is 
still more important. The problem in question is the difference between the geometrical shape of 
the film at the time the shutter was released and the geometrical shape it has when it is used on 
the plotting-machine. In particular, one does not know well the exact shape of the film at the 
time of its exposure inside the camera. 
Prof. F. Doyle: There are other errors than the radial distortion in the 
camera. In the y-parallax method, there is a considerable correlation between 
the first picture and the second picture and therefore, there may be errors' 
which act in the same way in the first picture as in the second picture, that do 
not show up in the computation. 
Consequently, of course, the grid method is the rigorous method but requires 
special photography on a special test field. The y-parallax method assumes that 
the camera’s pressure plate is flat and that the camera functions all right and 
gives a distortion curve under the assumption that the distortion is the dis 
placement in the radial direction from the principal point. 
Le Prof. F. Doyle indique qu’il y a d’autres causes d’erreurs que la distorsion radiale. Dans la 
méthode des parallaxes transversales, il y a des erreurs qui sont les mêmes dans la première vue 
et dans la seconde, ce qui n’apparaît pas dans le calcul. La » grid-method » est donc la méthode 
rigoureuse, mais elle exige un polygone particulier. La méthode des parallaxes transversales suppose 
que le plateaupresseur de la chambre est plat, que la chambre fonctionne parfaitement et que la 
distorsion est rigoureusement radiale. 
Prof. K. Schwidefsky: I would like to say that in my opinion it is not allowed 
to give to the two curves which Mr. Hallert showed us the name of distortion 
curves. In a physical sense these are two perfectly different things. Otherwise